A 48 hour wait from the ISP is pretty normal actually, so the backup router is the real fix here, not just a nice to have. The part people get wrong is thinking the LTE box kicks in on its own. Most of them need to be set up as a failover with a health check pinging something like 8.8.8.8, otherwise it just sits there while your main line is dead. And a prepaid SIM can run dry or get throttled right when you need it, so check the data cap before you trust it. The other thing is your booking system, if that lives on a local server it may still be unreachable from the outside even with the backup, so test the failover before the next outage hits. Cheap stubbornness is usually the reason these never get bought, so at least that part is behind you.

Learned that lesson on a 98 Accord I bought off a guy in Modesto, except my problem was the rear quarter panels. Looked fine in the dark but in daylight you could see where he'd packed them with Bondo and spray painted right over it. Now I run a magnet wrapped in a rag all over any car I'm thinking about buying, and if it doesn't stick, I walk. That trick alone has saved me from at least two money pits since then.