This is one reason why I posted the pic with the outside wiring. My network has a mixed environment of wired and wifi. Modem Zoom -> Linksys (6350) -> Actiontec (ecb2500c) 3x.
But that picture doesn't tell us how things are connected.. Or how you want them to be.
Three ECB2500c units.. Do the other two work together? Then I would say that the particular coax cable isn't connected to the cable network that the other two are..