Airport extreme no internet, though light is green

Hi,

My Airport Extreme won't connect to the internet anymore. It's light is green (no blinking or anything). It worked for years, but now it doensn't anymore.

My modem is working fine. I checked that, by directly connecting my iMac (by an ethernet cable) to the modem, and then there is a working connection to internet. So I concluded the Airport Extreme must be the problem. I even replaced the Airport Extreme by an (El-Cheapo) other wireless router, which worked like a charm. So the problem is indeed the Airport Extreme.

This is what I tried (many times and in various ways/order):

1. Rebooting the modem

2. Rebooting the Airport Extreme

3. Factury reset of Airport Extreme and setting it up from scratch

4. Disconnect all Ethernet cables to the Airport Extreme, except for the WANand mto my iMac

5. Disable Wireless modes

6. Downgrade firmware (and after this do not resolve the issue), upgrade to the latest firmware again

but none of it resolved the problem..

Suggestions what to try next??

Alte.