Hi there.
This is normal. Almost all manufactures equipment have a primary antenna port and a secondary port. The primary MUST always be used and the secondary CAN only be used if the primary is used. (the primary is normally the RH one if looking at the router from the front panel) The reason is one of bandwidth. The router is capable of Multi In and Multi Out, MIMO. This in effect makes the bandwidth (speed) twice as high when using both antennas compared with using only one. Check this website for your nearby tower: https://oztowers.com.au/Home/Query Carefully chose the antenna(s) and cable as well as the frequency of operation is a little involved and may involve a certain amount of experimentation if you are not a pro... Look at RFSHOP.com.au for more info on antennas... Best of luck.