AirPort Extreme problems

Tried to install my AirPort extreme and it's just flashing yellow, tried to restart it, but not working, holding the button for over 20sec, nothing happens, unplugged the cable and re plugged it, nothing happened and my imac doesn't find it

the cables are working, since I changed my Router to the old one. Why is it acting so weird

do I have to restart the modem?

Please help me out here! oO

yes

yes unplug your cable or dsl modem for about 10 min.
the modem remembers the internal device ID "MAC address" (nothing to do with Apple) of the device last connected to it and it will only communicate with that device.
Unplugging the modem for an extended period of time will force it to learn a new device ID.

Unplug everything from the wall (Power) and each other (data cables).
plug in the modem power to the wall.
Plug data cable between airport WAN port and modem.
power up Airport
you should be good now.

run airport utility on on your mac to configure the router.