iPhone drops back to cellular when on FaceTime call

Bit of a strange problem, but one which is very frustrating as I only get 1 bar of cellular signal at my house - which causes FaceTime calls to drop or become unstable.

I can be talking to my wife overseas on FaceTime (either just audio or video too), and notice that my XS Max will keep dropping off the wifi network when I am on the call. It may then sometimes reconnect to the wifi, but then will do so only door a few seconds before dropping again.

I’m not sure if it’s an iOS 13 beta bug, but has anyone else noticed this? I have done a “reset network settings”, but the problem is still occurring.