just to give my feedback not a solution :
Mi 10 Ultra, tried to get VOLTE and VOwifi since 2 days with no chances (tried all code and so)
today i used double SIM CARD (second one from another carrier) :
-First sim still cant enable both VOLTE and VOWIFI
-Second sim : Has both of them working great
NB: already have the option activated on the first sim and tried it on another phone and both working great
I assume its a compatibilty issue between my first sim carrier and the Mi 10 Ulra