Setting up EX7000 access point without wiring it to the router?

I’m not too tech savvy but from what I read you set the EX7000 up as an access point and connect it to the router using the Ethernet ports. However that’s not what I want exactly. I was wondering if I can connect the extender to the CAT6/Ethernet port(not sure what the correct name of port on the wall is called) on the wall in my room upstairs instead of hardwiring it to the router. The router is downstairs in my brother’s room. Our routers connect to the internet via CAT6 ports instead of coax since the ONT box in our garage acts as a modem (according to the Best Buy employees I’ve talked to). So instead of having a modem connected via coax and a router connected to the modem, we just plug in the router with out the need of a modem device. So I was hoping I could just do the same with the extender and not it connect to the router wirelessly or have it hardwired to the router itself.