Is the router's new IP address in the same IP subnet as the rest of your network?

The new router's IP (192.168.1.251) must be set to an address outside modem/router's DHCP-range (192.168.1.1-250) plus the DHCP-funktionality of the router must be desabled,otherwise it won't work as a WLAN AP for the main routers network.But I think this is the problem why I can't access the WLAN-Router anymore.But shouldn't there be an access to the router, for i.e. altering transmitting power etc. without purging it to factory settings?

The new router is not in the same network of of the DHCP range. And DHCP is disabled, if you would like to login, I think you can unplug the LAN cable from the main router, and connect your PC to the new router, set a static IP such as 192.168.1.100 on the PC, then login to the 192.168.1.251 page

doesn't´t work.