The problem appears to be that the DDNS service you are using is discontinued.
The announcement for this is available here:
https://eu.dlink.com/uk/en/support/dlink-ddns
If you don't want to have to click on the link, here is the entire web page:
"D‑Link DDNS
D‑Link Dynamic DNS (dlinkddns.com) service terminated on July 2nd, 2020.
The D-Link Dynamic DNS (dlinkddns.com) service has been terminated on July 2nd, 2020."
A reddit thread has a copy of the text of the email that Dlink sent to some customers:
https://www.reddit.com/r/d_link/comments/gmnlz7/dlink_dynamic_dns_dlinkddns_discontinued/
The email was a little more verbose than the support web page but the end result is the same.
Some dlink products might allow you to choose a different DDNS service provider than the one they terminated. You may need to update to the latest firmware to get those options.
Technically D-Link and how to make it function with DDNS is outside of the scope of OpenVPN Access Server. For further help, you should take a look at their own forums here:
http://forums.dlink.com/