In the same cabinet 1100ahx2 is measuring 31C and 750up is at 80C. Nanostation is drawing 5W and Rocket about 7W. Cpu is at 15%. Defective device or something else?
RB750UP can heat up only due to one reason - Power output on PoE-Out ports. Rest of the router generate heat next to nothing. Values you claim are nothing much if you are using 24V power supply provided with the router and cables are not very long (as there would not be huge voltage drop on those).
With 2 ports loaded, that anyway is a very high temperature.
I presume it is running latest RouterOS (or decently new release)