Please note that for the optimal performance for your router (GSM Module) requires you to use BOTH Main and Aux GSM Antenna.
The recommended theoretical minimum distance between Main and Aux antenna should be X, where "X" denotes the wavelength of the antenna operating wavelength.
It can be calculated by
So by example if you are using PR1KS210 (https://teltonika-networks.com/product/mobile-magnetic-sma-antenna/) with 4G sim then the Optimal Minimum distance will be 4.3 inches.
Hope this helps..!!