I conected an UTP cable from the airport extreme to an encore router, in the WAN entry, but I cannot get INTERNET in this new router.
I would suggest that before you try to connect the Encore router, that you verify that the Ethernet cable and connectors are working correctly.
That would mean temporarily turning off the wireless on a laptop Mac or PC, connecting the Ethernet cable to the computer, and trying to open your Internet browser as you normally would.
If you can open your browser and get a good Internet connection this way, then you know two important things:
1) The Ethernet cable and connectors are working correctly
2) the AirPort Extreme is working correctly and no other adjustments in settings are needed here
Now, it is up to the Encore router to connect using the Ethernet cable that you have provided. Unfortunately, the "how to" as far as the Encore product is concerned is really going to be a question for an Encore support forum.
It is possible....but extremely unlikely....that someone on an Apple support forum will have the specific answers about an Encore product that you need. You can certainly wait to see if anyone replies, but you will likely get a more informed and faster response if you post your question on an Encore support forum.
The following link appears to be for U.S. Support for Encore:
Support - Encore USA
Basically, the question that you need to ask on that forum would be how to connect the Encore router to another router using an Ethernet cable and how to configure it to extend a wireless network provided by another router.
The Encore product will need to be configured in Bridge Mode to "create a wireless network" using the same wireless network name, same security settings and same password as the AirPort Extreme network.
Good luck in your search for answers.
