5g showing doesnt *always* mean faster, normally means theres some sort of congestion, location of towers/ issues etc- wont really be possible to say why specifically, but it will be down to the provider itself, if its a new issue and wasnt there before than hopefully will be sorted soon. Theres a lot of factors beyond 5g/4g, but youre really only shown the baseline overview of what youre connected to. Be sure to ket your provider know