Setting up a Wired Connection for Xbox Gaming

I really need help here. I have an Asus RT-AC5300 router and I have Spectrum as my internet service provider. I want to have a more stable internet connection when it comes to gaming, so I want to wire my router directly to my Xbox using an ethernet cable. Now, my router is already currently connected to the modem itself via ethernet cable, and when I unplug it, the router says I have no internet connection. I know for a fact that people wire their internet connections without having to have the router connected to a modem, but I don’t know how. Is there anything I can purchase to have a direct wired connection to the Xbox from my router without having to worry about the modem? I currently use a wireless internet connection but it isn’t the best which is why I want to wire it. Any help, anything I can buy? Thank you!

 

So no matter what, I have to have the modem connected to the router in order to make the wired connection work? The modem is all the way in another room connected to the landline phone, etc., is there any adapter or anything I can use to get the connection without the router being connected to the modem?