Do you feel the difference between 180 Mbps or 250 Mbps? Do you actually need it or it's just about having higher numbers. I personally won't feel any difference between those, for normal usage. The only time you would need that is if you want to download large files often.
Auto makes the phone jump from 5g to 4g when it needs to save battery life, if you select only 5g, then it will only have 5g.
The reason might also be that across the world 5g is a relatively new technology, and carriers didn't implement it all correctly yet.
Where are you using your phone? In the US?