I've been working on this issue for most of this past boating season.
I started out earlier this year purchasing a Pepwave MAX BR1 MK2 LTE/WIFI router with a Peplink Mobility 22G antenna. I mounted this inside the boat using an ATT SIM card. It did 'ok', but not fantastic like I'd hoped. (I'd see about 30mbs). But then again, please consider that I'm looking to be at anchor, away from a marina and have great LTE/5G connectivity for work. I then added a T-Mobile sim to the unit and that helped. However... technology changes, constantly.
My current winter project is to solve this issue for me of "WFB" (work from boat).
I just bought a Pepwave MAX BRI Pro 5G (I pre-ordered it as it's brand new!) with a Peplink Mobility 42G high gain antenna that's also very new! I'm going to remove the Majestic TV antenna and replace that with the Mobility 42G antenna up on the mast. That location is high up on the boat, out of the line of radar, and should provide the best cellular data coverage possible.
I'm going to drill a 1 inch hole through the roof of my boat to route 7 antenna cables down the mast! (4 LTE antennas, 2 WIFI antennas (2.4ghz and 5ghz) and a GPS antenna. The Mobility 42G is a 7in1 antenna. The 6 cables all go into the MAX pro 5g router. The GPS antenna I'm going to connect to my Vesper AIS XB8000.
This specific router (MAX BR1 Pro 5G) has the capability of achieving over 400mbs of real-world cellular data connectivity. I don't need cellular voice as voice calls are ran over the LTE/5G data connectivity. So long as I have great data connectivity I'll have a fully functional phone.
The high-gain antenna (Mobility 42G) is specifically designed to re-shape the antenna pattern, to flatten it out, increasing power by reshaping that signal more horizontally (towards land) instead of up into the sky. You can't necessarily increase power, but you can take the power you have and concentrate it in a specific direction. Think of a flashlight, as you adjust the beam light from wide to more focused you get a more intense beam of light pointing a specific direction.
For the San Juans Islands, T-Mobile is going to be your best bet. I spent the money for 5G as T-Mobile purchased the 600mhz frequency in 2017. In 2019 they started deploying it. It's referred to as Band 71. It's unique to T-Mobile currently. They bought this frequency for rural America. The lower frequency is able to go farther than higher frequencies. With the newly acquired 600 MHz spectrum, which will broadcast as band 71, T-Mobile will be able to deploy far-reaching signals with greater bandwidth across the entire US.
Having multiple SIM cards is also a requirement. I have ATT and T-Mobile. This is necessary for diversity. When we head North into Canaday, and ultimately into SE Alaska, ATT is going to be the dominate carrier up there and will yeild better connectivity than T-Mobile.
The Mobility 42G antenna offers the following gain per frequency. Note, 3dBi of gain doubles the power.