Connect to a second router (UPS), when the power goes off

Configuration
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Let me try to explain the situation:

  1. Modem-ISP: 192.168.x.1 (DSL [Technicolor TG582n - WiFi Off]) -> Asus RT-AC51U (WiFi On) 192.168.y.1

  2. Modem/Router LTE 4G (Huawei B535s-235) 192.168.z.1

I have all my IoT devices (Google nests, smart plugs, Home Assistant [Rasp3b-Ethernet], lights, etc.) connected to the Asus router, using the gateway: 192.168.y.1.
While everyone, at home, uses the LTE connection (192.168.z.1); mostly because it goes about: 70/20Mbps while the DSL (192.168.y.1) connection goes 4/1Mbps.

Now, I’m thinking about using a UPS for the Raspberry with Home Assistant and the LTE Router; there’s no real need for it to stay connected for a long time, just so Home Assistant can turn off without damaging the SD card. But, as you can imagine, having the Asus router quite far away (on the other side of the living room) from the Raspberry, and having the LTE router next to it - both are hidden behind the 50’’ TV -, if the power goes off, I would need to create an automation, when the Asus router (state) goes off, so it should search the second router that also is connected to the UPS, send a notification about the outbreak and turn off without delays.

Any idea? :thinking: