Phone: Pixel 3a
OS: UT 16.04 OTA-22 Stable
Carrier: Ting (on i believe Verizon)
Started using: 4/23/22
I am getting much the same problem.
2G/3G/4G
On a fresh restart, I do have access to 4G. Text and calls and mobile internet work. However, this only lasts about an hour. After this happens, calls and text and mobile internet stop working. UT says it is "Searching" under Cellular Data. Sometimes it cuts out sooner. I can do something with constant internet access (play an audiobook or stream music) to keep it going. But the longest I have seen access to 4G maintained is about an hour. MMS never works.
2G/3G
After that happens, I can switch to 3G, and calls and text work all the time. But not internet. Not MMS.
I have noticed Wifi disconnects from time to time. I can select the router and reconnect, but it happens. But pretty rarely. I have never been able to use MMS, send or receive, on any setting above.