Cellular Failover

Needing some recommendations here. I'm a POS dealer for a bar/restaurant POS system and I'm looking for a viable affordable solution, if any, for doing cellular failover in the event the ISP goes down. I've checked into cradlepoint and it looks well but the problem is I'm looking for something more cost effective. I've spent hours looking at firewall specs that support USB cellular failover but out of all the models I can't seem to find any ones that support to many At&t or verizon cards, or they're just outdated cards. I've looked into Skyus that supports all major US sim's but from what I'm reading it mostly works for Marki's and that may just be too expensive for the merchant. The only reason I need a cost effective reliable solution because my POS system does not support offline processing. Does anyone know any info about Skyus working on other firewalls? I dont want to have to pay a cloud based subscription if possible. Thanks