Rules for a fail-over connection

I have the Firewalla Purple box. I also have an Raspberry Pi-based LTE modem that I use as a fail-over connection for my purple (over Wi-Fi). It works ok in general.

Since the LTE connection has certain data cap, I would like to be able to set up specific rules in my network while it's running on the LTE internet. For example, I'd like to limit, or slow down all video streaming, and gaming.

What would be the best approach? I know I can probably use "tc" for that, but how would I know when the box is running on the fail-over connection? I'd need to clean it up, when the main connection comes back.

Any pointers appreciated.

Mariusz