I would be surprised if Dell would cripple these cards and not offer 2,5 and 5 GbE speeds, but there definitely seems to be something wrong here. I do recall there being an issue with 2,5 Gbps not being advertised, and fixed with this commit in netifd:
It looks like the netifd in 23.05 should already have that though, since the one in the 23.05 release should is a snapshot from September. Could you test a main image instead of 23.05 and see if that works?