Hi, this is going to strike you as a newbie question, but:
The RB953GS-5HnT-RP supports triple chain, but is it possible to only connect two antennas and operate the built-in 5GHz wireless in 2 chain (2x2 MIMO) mode? If so, does it need to be configured in RouterOS, or is it just a simple matter of only connecting the two antennas?