Thank you for the answer; the situation is quite tricky because the router is placed on a slope of a narrow alpine valley and the only cell tower (as far as we now) is located at the bottom of the valley; we don't have a free line of sight to the bottom of the valley because of the orography, so the only thing that helped was placing an external antenna on a pole to elevate it. Since two days, without any other our intervention, the router has gotten the LTE signal with a proper strength, probably because they are dismissing the 3G network and maybe they improved the 4G cells. Anyway my question was about the use and the correlation between the main and the aux antenna, in particular if the two antennas must be installed in the same way to get the best performance and if the aux antenna can be useful in this kind of context or its purpuse is to get the full bandwith from LTE when the signal strength is good.