With 2 additional antennas on the CAT18 you will have better antenna performance. Possibly also additional bands supported by the CAT18 are providing strong signal,T-mobile…?
I notice the same thing with a CAT18 device connected to AT&T it will change from time to time. fluctuating throughout day between 2-4 bands carrier aggregation.
Have you looked at the inControl WAN Quality Reports? When LTE-A is active how do strength & quality look in those charts when you mouse over them?
If you are just using the included antennas on the Duo you could probably maintain LTE-A with external directional antennas in a better placement (roof/mast).