Detection of internet connection behind the router: Cable vs. Cellular (4G/5G)

I have this project on my mind: I've recently installed a 5G router in my office, to serve as a backup/failover internet source, to overcome main/cable internet outages. The backup Xiaomi 5G CPE router works well, and can activate backup internet line within seconds. (app. 2-3 seconds) My problem is, that the backup cellular line is a metered connection, is slower and less reliable. Therefore, I want to use it as little as possible. Therefore, I need to be informed when my office runs using backup internet connection. The problem with Xiaomi 5G CPE router is that although it switches to the ba

Question by Bunkai.Satori on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.