Some of us cannot get mobile data on our S20 FE 5G after Android 11 firmware update. I myself contacted Samsung sevevral times, reset network, reset phone to factory settings, but problem was not fixed.
Actuallu there are 2 possible fixes or workaround, until Samsung releases new firmware to fix the bug.
1) Remove simcard from sim1 tray and insert it to sim2 tray. It seems network mobile data works fine in sim2 tray. However, if you use SD card or 2nd simcard, this may not be an ideal solution.
2) While simcard in sim1 tray disable mobile network and go to
Settings-->Connection-->Mobile Networks-->Network Mode.
From there chose 2G and enable mobile data. Most likely nothing will happen. Then disable mobile data again and go back to Network Mode section and chose 3G. Enable mobile data. You should now get H or H+ sign. If you chose LTE/3G/2G (auto connect), it should give you 4G+ and should stay stable. I could not test 5G, as where I live 5G is not available. Please check and let me know if with this method you are able to get 5G?
