notes

This is mainly for future reference, unlikely to be useful for others.

First, reset the device. You need to push and hold the "reset" button for about 3 seconds.

Once reset, connect a LAN port on the main router to a LAN port on the other (not a WAN port), then, on a device connected to the main router, open http://192.168.0.1 in a Web Browser.

The next step seems to require a bit of luck or patience. You need to find the IP address of the second router. My method for doing this was selecting any "generic" devices (ones without a device name) and trying to connect to them. Of course, if a device exists with the name of the gateway, try that one.

Since you've been trying to connect to the second router, there should already be a tab asking you to login. Once you've logged in visit the LAN settings and turn off DHCP and set an IP address (e.g. 192.168.0.2) and restart.

It's possible that there is more, but hopefully that is all.