It seems to me that in this case the problem is with the remote access to the C6 than the Dynamic DNS resolution.

OK. Good to know that. What if you try to access the WebGUI of the C6 by its public IP address directly (http://<public IP address>:8080) - is it accessible ?

@dme3 While this is an excellent device, it's unlikely a new FW to be released, considering its status: You can still use it as an AP, but it's reasonable to buy a more recent device for a main...

L2TP Passthrough in ALG section is enabled, right ?

@hidalgo22 I presume the cable is connection between modem LAN port and the router WAN port. Remove the cable from the modem and see if you can regain an access to the router. Make the screenshots,...

What modem are you using ? Post a screenshots of the router WAN and LAN settings here.