I updated to IOS 14.1 on my Iphone 11 Pro yesterday and turned my Network Extender back on. So far so good, it will make and receive calls with the extender as well as show connected if you dial #48.
Previously it would show 4 bars until you dial the call then drop to one bar as it disconnected from the extender. The seemed to start with IOS 14.
I will update here if it stops again but hope this fixes it.