I made a wonderful mistake. It turns out I've been running v29 of DeluxeROM, not v31. I think I had some glitch on v31 at one point, tried downgrading, and then forgot about it. I loaded v31 today and even with the UK's "BTU" CSC, I'm able to use AT&T's VoLTE. I haven't been able to get WiFi calling to work yet, but that's not as essential to me.Where or what CSC should I use to try it.. as far as I remember in Oreo there is an option for this but now its gone..
I'm delighted I'll still be able to use this amazing ROM and wonderful phone for voice calls. Deep respect to AbrahamGC for all his efforts.