wifi-calling unreachable and changes back to weak cell signal

Hi,

I have a very weak mobile signal in my apartment, so I always use wifi calling at home. This worked perfectly with my old iPhone 6, now I use an iPhone SE 2020 with iOS 15 (current version 15.2 19C56) and the wifi calling is very unreliable.

The wifi signal has 4 bars and the speed test app measures 230 Mbps download and 277 Mbps upload speed on the phone.

The cellular signal has 0-1 bars.

- During a call, the phone keeps switching from wifi signal to very weak cellular signal and the call becomes unstable and drops. I use airplane mode to work around this problem. In airplane mode, I can make calls and the wifi calling connection is clear and stable.

- When the phone is in standby and wifi-calling is active, the phone becomes unreachable for incoming calls. When I press the home button and the phone's display lights up, the phone becomes reachable again for a while. This also happens while the phone is charging via a lightning cable.

The wifi router, network and cellular provider are the same as they were with my old iPhone 6. The only new component is the iPhone SE.

I would like to use my SE so that i don't have to activate airplane mode and so that I can be reached via wifi-calling again.

I reset the network settings on the phone, but that didn't help.

Is there a solution to this problem?

Thanks.