Thanks for your answer,
I will try and see how it works. From my understanding I have a combined cellular and VHF antenna in the top of the mast. There's a splitter which allows me to connect VHF to one port (which is used), and another port for cellular. The antenna is old and made for 900MHz (GSM 2G base frequency).
I am not keen to change antenna, and I guess that for LTE in the higher bands having the antenna high up will not do much difference, it helps more in the lower bands. So my thinking was that I use the Main port for a 4G antenna, and I connect the mast top antenna to the AUX port. The description seems to imply that both ports are used for reception of signals (otherwise it would not be possible to use the AUX for antenna diversity on 4G).