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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteIf a DIY phone has 2G/3G service but no 4G, work through the problem in order: check its network settings and data SIM, test the SIM and coverage, refresh the carrier configuration, then verify the modem’s LTE compatibility and carrier acceptance. If LTE data works but calls fail, troubleshoot VoLTE/IMS separately; it is a voice issue, not necessarily an LTE data problem.
First identify what is failing
“No 4G” can mean the phone never registers on LTE, or that it shows an LTE/4G indicator but mobile data does not work. Calls and SMS are separate symptoms. Note which services fail, whether Wi-Fi works, and whether another device using the same carrier has service in the same place. That distinction helps separate a coverage or registration problem from an APN/data issue or a voice-calling issue.
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Check the phone’s network settings
- Turn mobile data on and airplane mode off.
- If the build has two SIMs, confirm that the intended SIM is selected for mobile data.
- Set the preferred network mode to an option that includes LTE or 4G. Menu names and available choices vary by Android version, device, and modem software.
Pixel’s mobile-network guidance explains the relevant Android settings and troubleshooting, but its exact menu paths apply to Pixel phones, not every DIY build: Google Pixel Help: Connect to mobile networks on a Pixel phone.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsCheck the SIM and local LTE coverage
Follow the hardware maker’s instructions before removing or reseating a SIM; power the device down first if those instructions require it. Check the SIM for visible damage or contamination and ensure it is seated correctly. Recognition of the SIM alone does not prove that the account has service or that LTE can register.
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Try the phone in another location where the carrier is known to provide LTE. If possible, test the SIM in a known-working, compatible phone, or test a known-working SIM from the same carrier in the DIY phone. These cross-checks can help distinguish a SIM, account, or coverage issue from a device issue. HONOR’s support steps also recommend checking the SIM and coverage when a device has 2G/3G service but not 4G: HONOR Support: The phone/tablet has a 2G/3G signal but doesn’t have a 4G signal.
Refresh the connection and carrier settings
- Restart the phone, or turn airplane mode on briefly and then off, to prompt a fresh network attachment.
- Check the carrier’s current APN instructions for your specific plan and modem. If the device offers an APN reset-to-default option, use it rather than copying an APN value from another carrier or plan.
- If LTE or mobile data still fails, consider resetting mobile network settings using the device maker’s instructions. Depending on the device, a network reset may also erase saved Wi-Fi networks or Bluetooth pairings.
APN settings affect mobile data configuration; they do not make an LTE-incompatible modem support 4G. Android menus and reset effects vary by device, so consult the device-specific instructions. Google’s Pixel guidance describes its own network settings and reset flow: Connect to mobile networks on a Pixel phone.
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Verify that the exact modem works on your carrier
A SIM card cannot add radio technologies or frequency bands that the phone’s modem does not support. Check the exact modem or handset model and variant, its supported LTE bands, whether it is unlocked where required, and whether the carrier accepts that device on its network. Band support and carrier eligibility are separate checks: a device may support some relevant frequencies yet still be ineligible or only partially compatible, which can limit coverage or performance.
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Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →For US T-Mobile BYOD, the carrier states: “Your device must support the frequencies used by T-Mobile.” Its guide also recommends checking eligibility and VoLTE support, and warns that LTE coverage is not guaranteed for every non-carrier device or location. Those checks and requirements are specific to T-Mobile in the United States; consult the relevant carrier for other networks and countries: T-Mobile Support: Bring your own device (BYOD) guide.
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When comparing modems for a build, check the carrier’s current local LTE bands, exact-device acceptance and unlock status, whether the need is data or VoLTE calling, and the project’s interface and software requirements. Without the country, carrier, and candidate modem variants, there is no reliable universal ranking or band list.
If LTE data works but calls do not, check VoLTE and IMS
LTE registration and LTE voice service are not the same thing. If mobile data works over LTE but calls fail, ask the carrier whether VoLTE is supported and provisioned for the exact device and SIM, and whether the modem successfully registers with IMS. Configuration can vary even between models from the same vendor, so do not apply a generic IMS APN or modem command.
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In an LTE-only network, VoLTE/IMS may be necessary to place calls. Where 2G or 3G remains available, calls may instead use circuit-switched fallback. This distinction concerns voice; it does not establish that VoLTE is required for LTE data registration. Telenor IoT discusses model-dependent VoLTE configuration and APN/IMS setup in its own context, not as values to copy to another carrier: Telenor IoT: Test and Verification of VoLTE Service. Xiaomi explains the LTE-only calling distinction: Xiaomi Support: Why can’t the phone connect to a specific LTE frequency band on its own?.
Use the OURphone build as an example, not a universal fix
The OURS Project documents a Raspberry Pi-based DIY phone using a Waveshare 4G HAT with a SIM7600 cellular modem; its build uses a startup script to configure 4G internet. For a parts search, “SIM7600 4G LTE HAT for Raspberry Pi” is a useful phrase. This example does not establish that the HAT will fix an existing phone’s compatibility problem: verify the exact HAT variant, supported bands, carrier acceptance, and current availability for your project.
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Project documentation: OURS Project: DIY Open-Source Linux Smartphone.
What to give carrier or hardware support
When the basic checks do not isolate the cause, provide support with the exact modem or handset model and variant, country and carrier, SIM plan and account status, unlock status, and the results of your SIM/location tests. Say whether LTE registration is missing, LTE appears but data fails, or only calls fail; include any relevant status or error information. Those details let the carrier check account, device eligibility, coverage, APN, or VoLTE provisioning, while the hardware vendor can investigate modem and build configuration.
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