I also lost 4G connection after installing android 11. Network ist Deutsche Telekom, Provider klarmobil. Restart didn’t solve the problem. I hat to change to 2G in the Preferences (Mobilfunknetz/Mobile Network - Bevorzugter Netzwerktyp/Preferred Network type). Then restart. Connection with 2G (Edge) was successful. Afterwards I changed again to automatic (4G/3G/2G). Connection to 4G was successful.
It kept stable even after restart or changing from WiFi to Mobile Data.
Hope this can be helpfull 