There are a few options that we propose to try out if the situation allows it. The first option to attach double sided adhesive like 3M on the magnet antenna so that it can attach to the vehicle. We strongly recommend that you do a soft install first to make sure that the booster is working properly with a temporary adhesive that can be easily taken off and on so that it can be tested in various ways before permanently mounting the antenna.
The second is to create a grounding plane and mount it on your vehicle. You will need a piece of ferrous metal about 4” in diameter and a double stick adhesive. This will create a grounding plane so the signals can bounce into the antenna.
The last option would be to mount an antenna that has a grounding plane built into it. We would recommend the 4g Trucker mount antennas (304415 or 304414). These would require you to mount the antenna on a bracket attached to your vehicle so that the antenna can be mounted to it. A horizontal bar is the best way to do this since the OTR antenna was designed to attach to semi-truck mirrors.