Can't connect access point to Asterisk server

I've got a Zyxel P-1612HW-F3 with two FXS ports, I connected one phone to the FXS port on the back and tried to log it into the Asterisk server I have in my local network. But everytime iItry to do it, it says Registation failed , however I don't know why, nothing's in the log. I saw no SIP packet in wireshark also. It didn't work when I connected my cable into the Zyxel's WAN AND LAN, in both cases it got IP and the PC connected to it had internet, the IP of my server was visible from the Zyxel (through the ping section in webGUI), but no request, nothing in the Zyxel's nor Asterisk's log.

Question by pepa z depa on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.