networking – Network configuration for failover broadband

I’m not sure if this is the right place for these questions but I plan to build a new PC soon to run a trading algorithm I have been designing. I plan on using a 4G router (https://teltonika-networks.com/product/trb140/) as a backup should my internet connection go down.

Currently I just have a Sky hub connected to my home network acting as a modem, firewall and router for the whole house.

How can I go about connecting the new PC, backup battery (for monitoring) and 4G router to the existing network whilst only providing access to the 4G router for that single PC and the backup battery (so that I can monitor the power status remotely in the event of a power outage, which would kill the Sky hub too)?

The PC will be running Ubuntu desktop – I have toyed with the idea of using Ubuntu server since it will only be running scripts and will be headless for the most part but I don’t think it’ll be worth the extra hassle of not having a GUI.

Do I need a managed switch to connect those three pieces to the rest of the network via one of the existing unmanaged switches?

How can I configure the 4G modem to take over only in the event of the broadband connection being lost?

I am quite new to networking as you may be able to tell and can’t see anything specifically about this in the manual for the 4G router.

Any pointers appreciated!