Last year, around May, DynDNS.com has changed their root certificates, causing Dynamic DNS using DynDNS.com no longer to work properly. The error (Network Error) is displayed immediately after adding the account to your Sonicwall.
The easiest solution is to upgrade to the latest firmware:
Dell Support also offers an alternative, although this only works with more recent 5.9 versions:
Important for PPPoE connections on a Dell Sonicwall: with the current firmwares, the MTU is not automatically lowered to 1492 when using PPPoE as your WAN interface type. By default, it remains on 1500, so you need to manually adjust it to 1492.
This can be done by going in to the interface properties → Advanced, where you'll find a field to set the MTU.