Any time you change the hardware configuration of your network, shut the cable modem off, let it sit for 2-3 minutes (while you add or remove the other hardware). Then plug everything back in, and restart/power-on the network equipment.
The cable modem will "remember" the MAC ADDRESS of the last piece of hardware it was attached to. If you leave the modem powered on while adding a router, the MAC ADDRESS retained by the modem will be that of the NIC (Network Interface Card) in your computer. The router has a different MAC ADDRESS, and the modem won't "talk to it".
See this thread: »[ Brighthouse] Help with router on Roadrunner please for reference.
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