Wifi calling lost each time I leave the house

I need to use wifi calling on our broadband satellite service as we have no phone signal at home. (In a remote area of Australia). Since upgrading to ios 15 (& now on latest update 15.5.1) ios takes up to 24 hours to be recognised by my phone. Once it has been recognised it works fine, but if I take the phone away from the house (& thus the wifi signal) it takes up to 24 hours to be recognised again after I return home. Our nearest 'town' - a small indigenous community only has a 3G (yes Three!) service, but I cannot see why having the phone connected to the 3G service (for calls) whilst in town should affect my phone's ability to recognise wifi calling immediately once back home where we have no cell phone signal.

I have tried the full range of 'remedies' suggested online for wifi calling not being recognised & nothing has made any difference. (including deleting network settings, restarting, hard reset, switching aeroplane mode on & off, wifi calling off & on etc etc).

I also installed a brand new wifi router but still no difference.

Wifi calling used to be very reliable on this phone & not having this facility when I need it is a major problem for me. The one thing I have't done from all the online suggestions I've read is to contact my phone service provider - as when wifi calling on the phone is recognised it works just fine, so very much seems to me a phone problem & not a network problem.

If anyone can offer me any further help I would be most appreciative.