I have TP-LINK TD-W8980 router. I was using it as a DSL router. Now I switched to cable and connected its WAN interface to the cable router.
I'm fail to find the relevant wan interface and to configure the bridging towards the lan instead of the dsl.
All I succeeded to dig is this line in dmesg:
lantiq,xrx200-net 1e108000.eth eth0: port 4 got link
In most cases, you should simply reset the OpenWrt to defaults.
Have you done this?
You would do something like this, assuming the firmware is still swconfig based.
Basically you need to split off one of the Ethernet ports to be a separate VLAN, then attach that VLAN to a wan interface of proto dhcp, then connect that port to your cable modem. The other 3 ports are still usable for LAN devices. References to the dsl system should be removed (or at least left detached from all network interfaces) when you're not using the internal DSL modem.
Just be aware that lantiq vr9 maxes out at ~85 MBit/s without software flow-offloading and around 110 MBit/s with software flow-offloading enabled. Considering that most cable connections tend to be faster than that, I wouldn't spend too much time on getting a TD-W8980 xDSL 'modem' working (although mk24's suggestion is correct and would work, within the capabilities of your device).