As a number of other posters have already told you your phone does not support 5G (neither does my iPhone XS). However, there is no reason why you should not see 250Mbps if you are in an area where signal is good and the tower is not overloaded. I saw 260Mbps when I ran a test after signing up.
@Joybellswj1 which Nework mode settings do you have? 5G Auto or 5G On?
5G Auto will allows iPhone to drop back down to 4G to svae battery most the time when the extra speed is not needed
if you want ot test your 5G, change to 5G On and do a speed test. But remember, it could drain your battery faster