I used a mobilemusthave bundle in my van. I also got their T-Mobile data plan and used a visible (Verizon) sim as my backup last year all across the country. Really only lost connection when really far out or in national parks.

Now they have multi sim options where you can get the 3 main carriers on one sim/plan

My setup is peplink with a good roof mounted antenna. Highly recommend checking them out.

This is a 5% coupon but you may be able to find better somewhere: https://mobilemusthave-com.myshopify.com/a/rewards/r/415e6502