set up for LG tv, Apple TV box

I have a LG TV and just bought Apple TV 4m box and am installing ,

My Modem is Spectrum and router is my own for internet and other devices and works well.

My question is: the modem only has one Cat5 ethernet connection and have it used for the router to be connected, how can I use Apple TV connection on back of box and going to connection on back of modem.

I need my Internet connected to my router