Find IP address for SiteCom router

I have one router/modem from the ISP, a Nokia XS-2426G-B on IP address http://192.168.1.254. If I go there in the browser and login, I can setup a WiFi access point.

On the second floor I have another router, a Sitecom WLR4100 X4 N300. Login to that router is supposed to be possible via 192.168.0.1 or http://sitecom.router, but that fails. The second router was only ‘necessary’ for defining a second WiFi access point, it has network cables plugged in from the 1st router and from two Windows PC; essentially functioning as a switch only (WAN port not used).

After some power outages (for testing the mains groups) this setup no longer works.

If I look at the devices detected in the first router I see the 2nd router’s MAC address – it has dynamic IP 192.168.1.65:

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If I go to that IP, the connection times out. I tried http://192.168.1.65/login as well.
How can I get access to the 2nd router from my browser?
I still have the admin login for it.

Notes:

  • This suggested question did not help me.
  • I also tried 192.168.100.1
  • Pinging 192.16.0.1 or 192.168.1.65 fails
  • All my attempts are from a PC hardwired to the SiteCom router
  • Internet access works, also via the WiFi access point on router 1
  • The setup worked perfectly fine until one week ago when I started meddling with the mains supply. Both routers have been rebooted since.
  • That includes printing to a HP Laserjet over WiFi, which is also no longer possible. I may dig up a cable and connect that.