We use the Peplink MAX BR1 Mini LTE | Industrial-Grade 4G LTE Router | MAX-BR1-MINI-LTE-US-T at about 80 customers. We like it. It's easy, it works. Verizon charges anywhere from $10 to $20 per month depending on how you get them to include it.
The hardest part of any failover system (except the integrated solution from Comcast) is the router setup. We use all Sonicwall and it works perfect for FailOVER, but every once in a while it doesn't failBACK.
I even carry one of these with us on vacation when we travel to remote places where cellular IS available but decent copper broadband ISN'T.
This plugs into you network via ethernet, or it can be a WiFi router. (or both!)
Here's a link: https://smile.amazon.com/gp/product/B01MR5YKF9/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1