Fix China Telecom 4G Roaming on AOSP ROM by Changing APN

Since the support life of my OnePlus 8T's official ROM is about to end, I flashed Nameless OS, a Lineage OS based third party Android ROM, onto my phone. But after flashing the ROM, I found that my China Telecom SIM card cannot roam on the 4G network of local mobile service providers, only 2G or 3G work. Since the local providers are recently shutting down 2G and 3G networks, the roaming cellular signal strength is really bad. I experience a high latency on receiving or sending messages, nor can I use VoLTE to make calls normally.

I tested other Lineage OS based third party ROMS, and experienced the same problem.

After numerous attempts, I found that the problem seems to be with the phone's APN settings. I use the term seems to be because while changing APN settings fixed my problem, I have no idea why this works, nor am I sure this is the correct way to fix it.

Here are my steps to resolve the problem:

  1. Enter phone settings - SIMs - China Telecom - Access Point Name settings.
  2. Select "中国电信NET设置" (ctnet, China Telecom NET Settings) to manage detailed options.
  3. Change APN Type to: default,supl,dun,ims
    • The original value is default,supl,dun
  4. Click the three-dot menu on top right, and select Save.
  5. Select the radio button on the right of ctnet.
  6. Turn off the phone, wait a few minutes, and then turn it on again.