I can get to the web interface, but no internet connection – this is all over a LAN connection.
You haven't said anything about why you suspect the router and not your cable/DSL (??) modem. (I'm guessing that you use one or the other).
Since you have been able to complete a number of firmware changes to your router it sounds like it is working fine. Consider what I think you are implying. The firmware seems to work fine in the router. So there would have to be a failure to the hardware for the WAN ethernet support which would not be reported in the router's log. While anything is possible, the router would not be the first device I would suspect.
First thing I would do is to reboot the cable modem. Wait an appropriate period of time and then try reaching the Internet again.
Since you can reach the router's admin page, see what it says about the status of the external WAN/Internet connection. Was the router assigned an (exteral) IP?
Also look at the router's log. If the router has filters for the log, set them to tell you everything. Then reboot the router and check the log again to see what happened when the router booted and attempted to get an IP address from your ISP.
You should also check to see if you are reaching the Internet but unable to view any web pages because you are have DNS problems. Try pinging from your router (should be available under "Tools") one of the OpenDNS servers,
184.108.40.206. If this works, but you can't open a web page I would suspect a problem getting results back from your ISP's Domain Name Servers.
One final thing you might try to test out the router vs the modem is to power off the modem for a few (3 to 5??) minutes, connect a PC directly to the modem, then power it up. See if you can reach the Internet then.
Two notes on the above suggestion:
- I suggest you do NOT leave things this way for very long even if it works. Having a PC connected directly to the Internet is an open invitation to others to try to hack into your system.
- The reason I suggest leaving your modem powered off for a few minutes is because commonly an ISP will note the MAC address of the router or PC which is connected via the modem. The ISP will refuse to (immediately) assign an IP address to a system with a different MAC address. Typically if you wait a bit the lock-out times out and then any MAC address is allowed.