ummm most people don't get there internet signal (which is what a modems for) from Ethernet cable, Ethernet is a computer data cable (prob not the best term but it'll do).
Anyways what I'm trying to say is you don't need the modem if your "internet is coming from and ethernet cable from the wall" and the only reason your internet is coming from the wall is if you live in some uni complex, hotels, motels, block living with shared internet pretty much.
anways what a modem does is converts physical signals to computer data. And ethernet is not used as a form of signal cabling (prob another bad terminology lol), which means its already coming from a modem somewhere else.
If you plug your computer into the ethernet cable your internet will work providing its activitated/connected on the otherside of the ethernet cable (which will be in the same building as you somewhere). and if you want to use the switch, plug ethernet into switch and computer into ethernet.
hope this helps