Yes its possible to choose a specific band .
You can use the manual mode and after you are in manual mode you can select the band you want to use.
Differences in Rutx11 and Rutx12 are :
1)Rutx11 is 4 Ethernet ports, Rutx12 is 5 .
2)Cellular speeds up to 300 Mbps in Rutx11 and in Rutx12 cellular speeds up to 600 Mbps with dual simultaneous LTE connections.
Further you could check more here :
Have a good day.