said by More Fiber 
:You don't need to have the RJ45 enabled on ONT to use the Actiontec as a router.
It makes no differences if the Actiontec is connected via coax or cat5.
If I understand what you are asking, is this:
Connect the 2nd flr router's WAN port to a LAN port on the Actiontec.
Leave the Actiontec on it's default subnet of 192.168.1.x.
Configure the 2nd floor router to use the 192.168.2.x subnet.
Without explicit routing entries, PCs on on the 2nd floor won't see PCs on 1st floor and visa-versa.
I don't think that's what phat_h wants, but the fact remains that that is what he'll get.
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