In areas with weak signal it can help to use an External 4G/LTE Antenna to help connect to the network. This should be connected to the External 4G/LTE Antenna. 

It is important to only enable this setting when an external antenna is connected, using this without an antenna may damage the internal electronics of the modem. 

You should only use antennas intended for 4G / LTE applications, other antennas, such as the 2.4Ghz range extender, may result in worse performance.