I've been using a Pepwave BR-1 modem/amp/router ( peplink.com )with two BoatANT antennas (cell and wifi) for several years; there have been only a couple of times without any usable signal. I'm on Verizon; perhaps an additional AT&T sim as fallback would have worked. The device will hold two sims and switch between them.
This is an internet device and is NOT a phone "booster." We have zero interest in yakking on a phone, texting or using the internet on a phone screen.
However, though, we can of course use phone internet functions via the Pepwave and also use a app called Viber which provides voice-over-IP capability when we really want to talk and there's insufficient phone signal. An alternative is of course Skype.
The Pepwave is also a great wifi repeater. Some new models don't have have the repeater.