[MT-AU Public] Antennas for Sierra Wireless 320U

Hi All, we are just about to deploy some routerboards with Intel NUC and 4G Modems for OOB units. As the units will mostly be installed in MDF rooms we need to boost the signal in a number of locations. Does anybody have any real world experience with any reasonably compact antennas that could be mounted on the outside of a rack which will improve the 4G signal fairly significantly ? We will be connecting them to the Sierra Wireless 320U USB dongles which have two antenna connectors so it will have to be compatible with that. I would appreciate any suggestions Thanks Paul