I'm on virgin mobile who use O2 network and also 2nd SIM is EE, when I'm around Wolverhampton area, I have to disable 5G, and only use 4G, the 5G masts although I get full signal do not allow me to make any calls and I get zero data, just as if I have no signal and no data connection at all, connect to the 4g and it works fine, it's always been this way, even before the beta, my Sims are both 5G enabled and obviously the S21 ultra is too.