How to switch the router to a usb modem in case of a drop in the Internet?

Question

Computer networks

Hello.
Prompt as it is better and on what to organize such network.
There is a wi-fi network in the room with internet access.
You need to raise a router that will connect to this network and receive the Internet and distribute it within your wifi network. But in the event of a drop in access to the Internet (let's say a Google site ping), switching to a usb modem.
What equipment to use so that it can perform such a task and at least a description of how to set it up.