Home Internet and wifi extender

I've tested with success using my two Apple AirPort Extreme routers. First test was a short ethernet cable to the first AIrPort and wireless to the second AriPort. That worked fine. Then I set it up connecting one AirPort to the Gateway with a long Ethernet cable (in the walls in my house already). That works fine too. No noticeable signal loss. I don't see anyway to connect any router or extender to the Gateway without an ethernet cable.