Short answer to the original question (that was asking for something "dead easy"): no there isn't.
You could find something good enough for you if you were willing to give up on one of the features:
- dead easy
- pure software solution
If you accept something that's definitely not dead easy, but that gives you much power in return, then I'm sure you've already found several possible solutions, one of which could be configuring Linux as a router with failover.
On the other hand, if you want it somewhat easier than becoming a network administrator (but "easier" is quite subjective anyway), and you can accept a new piece of hardware, then there are several routers out there that can fall back automatically to a mobile connection when the main upstream connection fails. They implement that in software after all, but you need the damn router to run that software.
EDIT after the OP edited the question, changing "dead easy" into "that can be walked through easily":
Keep in mind that "easily" is still very subjective and that the problem itself is all but easy. That said, I'd recommend the ASUS DSL-AC88U (a.k.a. AC3100) router. It runs AsusWRT system software which I find very intuitive and well designed.