Cisco modem won't allow WRT54G downstream

[TwoDogsTwoCats2]

I just hooked up Charter Spectrum cable, and they provided me with a single port, wired modem (Cisco). I was able to hook up an older Windows 7 laptop, successfully (that one had an ethernet port), but when I tried to hook up the modem to a Linksys WRT54G router, that I had flashed with DD-WRT firmware, to turn it into a switch, i get a flashing yellow light on the "Link" LED on the modem. I didn't know that Charter wasn't furnishing a wireless modem, so I had to stay on my ATT - DSL until I got a new Synology RT1900ac router. I got the router, installed it, and then tried plugging the Linksys in, to no avail. I get 4 green lights: Status, 2.4G, 5G, and WAN, but get a yellow light on the wired port indicator, when I plug the Linksys cable into the Synology router.

I have the Linksys behind my TV, to feed my TV, my XBox, and my DirecTV.

I had been using the Linksys with the ATT modem, for quite some time, without any issues. Is there something special about the ATT modem (I had both a Netgear and and before that, an older Pace, both wireless, with 4 ethernet out ports, that worked just fine).

Does anyone have any ideas how to get the Linksys working?

Thanks!

 

[The Paladin]

but when I tried to hook up the modem to a Linksys WRT54G router, that I had flashed with DD-WRT firmware, to turn it into a switch

you cannot connect a switch device directly to a modem, you need to put a router in between
ISP MODEM --> Router --> Switch --> wired devices.

the reason why is the port on the modem can connect to a regular network device directly or a wan port of a router but not to the switch network port of the switch, technical reason is the single cannot be split at the modem with a switch it requires management, or a single computer connected directly to it.

you could flash your wtr-54G back to the Netgear bios and use it as a router, or set your DRWRT to be a router instead of a switch. (connect pc to it and change it back or press reset button while powering up holding it 5 seconds it will reset to default router mode. configure it for your new modem. and be done.
note when you connect your router to the modem after putting a computer on it, you need to turn off the cable modem and turn it back on to reset the connected MAC address so the signal is not locked on your old MAC address.
or configure your router to use your POC mac address thru the wan port settings.