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Not quite sure how you got the router working, even temporarily, as an ADSL combined router / modem has no means of speaking properly to a cable modem. Simply put the WAN side of the ADSL router is hard wired to the ADSL modem.
I can only imagine that temporarily you had the modem connected to one of the 4 LAN ports, and one computer only connected to another LAN port. That might have confused things enough that the router acted for a while simply as a switch (junction box).
Sorry, but I've got to say it, you need a different cable compatible router
