LB2120 and external antenna locations

I have an LB2120 LTE modem positioned in line of sight of a 4G mast through a window. To this I have attached an extenal Poynting antenna, also in line of sight of the same 4G mast.

My quesion is, will the modem automatically default to take its signal from the external antenna (which should be the stronger), or do I need to move the modem so that it is no longer in line of sight of the mast to ensure the modem takes its signal from the external antenna?