FLEX ethernet connection

Everywhere I read, it says Flex can be connected to the internet by ethernet cable.  I have no WI-FI, so I assumed I would be able to use FLEX without a problem.  This has not been the case.  I was on the phone for a long time with an XFINITY tech that didn't get anywhere.  He sent me a new FLEX box.  When I connected it directly via ethernet cable ( I had used a splitter before) I noticed the top blinking  light on the modem was red.  

When I plugged in my PC to the same port it blinked green.

This tells me the box is not connecting to the internet via the ethernet cable.  If someone knows how I can get this fixed I would be most thankful.  Otherwise. please let me know if I have to return the boxes.  I am not going to purchase a WI-FI router just to watch the PREMIER LEAGUE.  

I am using a personal modem:

MOTOROLA 8x4 Cable Modem, Model MB7220, 343 Mbps DOCSIS 3.0

Than you for your time'

CodyComm