So the correct answer is a IP address that is in the subnet of the main router but not included in the DHCP range. You would have to check the DHCP on the main router. You can likely use 192.168.100.250 until you figure out the correct one. Most device give out DHCP addresses from the bottom and using a high one you likely would not get a duplicate.
What do you mean "extender". You can likely hook that router up as a AP connected to the main router via ethernet. If you intend to connect the secondary router to the main router via wifi it is much more complex. The secondary router would actually have to have a feature that allows this. That is not a common router and I can't find the manual easily. You need to read...