The mast and router might be using NSA 5G so despite there being 5G in the area the router still needs to connect to the 5G mast via a 4G connection first to negotiate its link for 5G. This isn’t happening because the 4G signal needed first is coming from the internal antennas that may have weak signal and possibly not connecting to the 4G mast that has the 5g ENDC uplink for 5G, therefore your not getting 5G on the T9 external inputs because of this.