The RUTX12 is a dual CAT6 Modem device. LTE-A CAT6 defines a maximum of two carrier bands which can be added together and no more. You are comparing something very different. The specification page for the Netgear Nighthawk M2(if this is the router you have?) states:
Supports 4GX LTE Advanced Category 20 with up to 5-band Carrier Aggregation
Using the poynting 2x2 MIMO is the correct idea, depending on your location of use the OMNI version is best for 95% of users.
Note also, the design premise of the RUTX12 is more about seamless failover than increasing bandwidth.
Also note 4G/5G suffers from network contention and speed tests even with a perfect antenna setup installed in a clean RF environment you will see a large variance in speeds based on time of day/number of users connected to cell.