One of
these, a T-Mobile SIM card and an external antenna.
Works great for coastal cruising in the Pacific Northwest, I have been setup that way for about 5 years.
Or, if another cellular provider has better coverage in your area, use them.
Be sure to check with the cellular provider about data limits. I believe for T-Mobile you will want the small business plan, otherwise there are caps at like 50GB or something and it slows down.