The problem seems to be that there are some old connection states saved, even if I clear the connection table. I have every 24h a disconnect and after the disconnect the asterisk doesn't connect to my SIP trunk anymore. I can only reconnect if I disconnect the asterisk for 5-10 minutes or if I reboot the router. I used wireshark and can see that asterisk is sending REGISTER packets, but I never receive a packet from the SIP trunk.