I sail out of channel islands harbor regularly. On trips to the East End of Anacapa Island I lose cell reception at about 7 miles from CI Harbor. Going to Santa Cruz Scorpion Bay I had cell reception at 16 miles from CI Harbor. I had Verizon from 2019 to 2021 on these trips, and the service was not so good, the phone was an older iPohne SE. I now have T-Moble on an Iphone 13 and service seems way better. I have not been all the way to Santa Cruz island yet as I ran out of daylight and turned back home. I hope that helps
Dano