My phone (Xiaomi Redmi Note 10, Android 11) spontaneously disconnects from the mobile internet after being connected for a couple of hours, and it won't automatically reconnect. Can stay unconnected for however long. After I switch to the flight mode and back, it reconnects for some time, then loses connection again.

Is there any way to diagnose what is the problem? Maybe to find some logs from the network hardware.

I thought this might be due to a defective SIM card because the same problem happened to the phone it was previously installed in, but I had SIM card reissued at a mobile operator's office. The same problem is there with the new SIM card. What's the chance it's defective again? ¯\_(ツ)_/¯

UPD 2:

Turned out that 4g connection always disappeared during a call, and not just spontaneously as I originally thought.

UPD 1:

A little clarification: when internet is Ok the signal level has a 4g label, then in an hour or so the 4g label spontaneously disappears, and so does the internet connection. But the phone does calls and receives them.

Thanks to Robert I managed to get logs by

adb logcat -P "" # and then after waiting for the internet to vanish
adb logcat -b radio

A lot of messages say 2224 at the presumable time of failure such as [0]onDataSetupComplete: error apn=internet.tele2.ru, cause=2224, requestType=NORMAL

This describes it as "SIM was marked by the network as invalid for the circuit and/or packet service domain." How could this happen after sim worked fine for the previous 40 mins? Is there anything I can do to prevent it?

Here are the logs, but after seeing what a mess they are, I wouldn't expect anyone to look through them. But if you actually do, the logs start with turning the airplane mode on and off, then a lot of messages start at 22:31 (where the loss is probably is) then by 23:01 there is definitely no internet connection.

Any help is much appreciated.