since you mentioned Main 1/2 and Aux 1/2 antenna connectors I suppose the connectors on your products look like this (ignore those RED circles):
To get 5G MIMO operation you should connect the direction antennas to ports (Main 1A, Aux 1A). These two ports are TX/RX capable. The remaining ports Main 2B and Aux 2B for this product are RX only (diversity). For your case to operate in LTE bands 12 and 14, please connect to Main 1A and Aux 1B.
Please keep in mind that this configuration, described above, is valid for this product only and this particular SKU.
For newer models we have started new naming and we simply name antenna ports as A, B, C and D. The issue is that every cellular modem vendor has a bit different configuration, thus not to confuse end users we changed the naming as well.
Lastly, probably you already know Peplink has antenna series as well. The directional antenna you are looking for will be ready roughly in August (2x2) and September/October (4x4). The antennas we design cover almost all LTE/5G bands (the exception is with 400MHz bands which are quite rare). I mention this, since if you don’t want any headache and want antenna which is future band proof (LTE or 5G) then you might considering trying Peplink antennas as well.