Establishment Watchguard

Open Policy Manager and choose the menu item
Network -> Configuration
and then the tab Wins/DNS

Register the DNS-Server

Change the tab into Dynamic DNS

[x] Enable Dynamic DNS
User name: HOST-ID (Is shown to you when name is created.)
Password: (Is shown to you when name is created.)

Service Type: dyndns

Now Change the tab into Interface and click the configuration of the external Interface.

Remove the tick “Negotiate DNS with PPPoe Server” in den "Advanced Properties" of the external Interface.