Hi Jim - welcome to the forum!
Lets get to the bottom of this for you. Firs place to start is signal readings. Please login to the BR1 mini and share a screenshot of the RSSI / RSRP SNR and RSRQ readings from the cellular connection.
What type of boat? What is the primary construction material?
External Antennas normally do help, but lets work out if signal is the problem here first and then look at your options.