Hi Andrew
Thanks for the guide. It worked for me.
getprop persist.vendor.somc.cust.modem0 returned S20.1
There was no directory called this.
I found a config for EE in UK on the phone
/vendor/firmware_mnt/image/modem_pr/mcfg/configs/mcfg_sw/generic/euro/ee/vlvw/uk/mcfg_sw.mbn
I made the missing directory - mkdir /vendor/oem/modem-config/S20.1
and added modem.conf file with the contents
mcfg_sw/generic/euro/ee/vlvw/uk/mcfg_sw.mbn
Now showing as VoLTE registered.