Just out of interest I went through the procedure of generating router root and self-signed certs. I now want to revert to the defaults but there does not appear to be any option to delete the self-signed one. Contrary to the manual, the cert appears to be saved in a config backup, so a simple reset to factory settings and load a config backup doesn't work. Factory reset then manual restore would be a right pita. Any ideas anyone?

With the Trusted CA Root Certificate, there is an option to delete that.
The self-signed certificate though, I can see what you mean, there's not a way to clear that.

You can generate a new one with less details though, these are the default values:
C=TW, ST=HsinChu, L=HuKou, O=DrayTek Corp., OU=DrayTek Support, CN=Vigor Router

As a pedant I would say fewer details, but thanks admin, I'll try that.

My reason for asking btw was that every couple of days my iPhone X would decide that it no longer trusted the Draytek certificate and would revert to a black hole IP address. Frustrating.