OK all you need to do on the spare router (assuming some IT knowledge here)
-Set its management IP address to something which is NOT in use and is on the same subnet as your network - for example if its IP address is currently 192.168.1.1 - set it to 192.168.1.2 - because 192.168.1.1 will probably be used by your main router. Set it to something outside the DHCP range on the main router.
Ensure the management IP is on the same subnet. For example if your main network is currently 192.168.0.1, you will need to change the IP of the spare router to 192.168.0.2 as opposed to 192.168.1.2 - if that makes sense. Poke around in the settings to find it.
-Disable WAN services etc, not needed.
- Can you run a cable between both the routers? This will avoid using bridge mode. If you can't, then you will need to configure the wireless mode to bridge/repeater, and you will need to make the SSID the same in that instance.
If you can get a cable run between them:
-Set wireless channel to a 1, 6, or 11 - one that your main router is not using, this will avoid interference. You can either set the SSID to the same as your main wifi (and hope the wifi cameras will choose the nearest AP), or, set the SSID to something different and then you can configure the wifi cameras onto that specific SSID. If you set the SSID the same, you must make the encyption setting the same.
Bit rushed but try that see how you get on, any questions come back and ask.