Hi Tom
In my case it was a variety of things.
First I would make sure that you earth the unit on the same point than your communications equipment (there should be an earth busbar on the bridge)
Second, I would check with your phone, using network cell info app, how good is the signal there and what speed up/down you get.
Sometimes you can have all the equipment wired correctly, but still get a low SNR value because of surrounding interference (this was my case in Miami).
As the others say, the shorter cable length from the BR1 to the antenna, the better signal you will get.
I would also recommend making sure your SMA connectors are tight always, as onboard yachts the vibrations can get a bit too much and loosen them up (better use a velcro tie and tie it up nicely).
Let me know how you go and see if we can get it improved.