RUTX50 employs all four antennas as main, meaning that there is no separation of connectors between 4G or 5G.
Whether to operate on 2x2 or 4x4 MIMO is determined by the modem, based on the received signal characteristics from all four channels, thus selectively connecting a 5G directional and a 4G omni may not result in your desired output. The again, you can still try to connect the Poynting antenna and attach omnidirectional ones to the remaining connectors.
Besides, the device would still operate in either 4G or 5G at a single instance. I would suggest to test setups for 4G and 5G and see, which yields better performance.
Also, RUTX50 uses a LTE Cat20 modem and, in order to fully utilize its capabilities, all four antennas are preferred with identical features.