It’s a common issue with custom ROMs. Samsung phones cannot get working VoLTE on any ROM that isn’t based on OneUI, and many models from other manufacturers don’t work either. There are tools like Pixel IMS that can theoretically allow you to manually enabke VoLTE, but I’ve never seen it consistently work on anything other than Google Pixels.
It’s a common issue with custom ROMs. Samsung phones cannot get working VoLTE on any ROM that isn’t based on OneUI, and many models from other manufacturers don’t work either. There are tools like Pixel IMS that can theoretically allow you to manually enabke VoLTE, but I’ve never seen it consistently work on anything other than Google Pixels.