WiFi dropping when on FaceTime and going to 3g

When on FaceTime my wifi regularly drops out and goes to 3G. It will only reconnect if I hang up which can sometimes be inconvenient. I can’t turn off the mobile data/cellular option for FaceTime as I need to use it out and about! At home I live in a 3G/4G black spot so it just says poor connection and then I have to hang up, reconnect again and it’s a pain!

I’ve had iPhones before and my partner has one too and it has only ever happened to me, on my iPhone XS Max! Please help. It’s driving me crazy!