How to find the IP address for a Sitecom WLR4100 X4 N300?

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 Wi-Fi 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 testi

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