My current cable router/modem has 4 ethernet inputs, all of which are in use. I want at least one more. What are some ways to do that?

I already have and I am using plug-in to home wiring wi-fi extenders that have an ethernet input, but the last one I have (that would solve my problem) is refusing to sync, hence, my query here. Also, I do not want to change out the current router/modem.

On a Mac since 1984.
Currently: 27" iMacs, Macbook Air, macOS 10.15.x,; iPhones, iPods and iPads galore!