Backup Internet line based on 3G / 4G modem, which option is best?

Question

Yota

Good day! Faced such a problem today - the provider fell off and the work began throughout the organization. Therefore, the issue of a backup Internet line was raised, based on a 3G / 4G modem.
What can you advise?
Ideally, it should be like this:
1. The main Internet drops
2. The router immediately automatically picks up this moment and switches to the Internet from the modem
What I know at the moment:
1. There are openwrt and dd-wrt with support for balancing the Internet connection.
A natural question arises - what is easier to maintain and on which routers is it best to take this miracle?
2. There is an Iota modem, ours. It is a 100% worker and once helped out (they hooked it up to the Mac, and in the Mac itself they registered the local address of the router, having previously turned off the router). But here another question arises. We have 50+ people in our offices. And to use one single modem to access the Internet for everyone ... is doubtful. But maybe I'm wrong.
Yes, even if the modem itself pulls out, then where is the guarantee that it will not fall off either? That is, it would be desirable to set up a double reserve: the main Internet, iota, some other modem, in order to protect yourself for sure.
Pulling the cable of the second provider for some reason, alas, is not an option.
It is necessary to take into account the moment that there is not much time for setting up - a couple of hours before the start of the call center's working day.
What can be done in this situation?

Answer 1

Pulling the cable of the second provider for some reason, alas, is not an option.

The only correct option.
Do you want to put a call center on a 4G whistle? Hard.