IE provides an override at the bottom of the browser when this happens. AFAIK, Firefox does not and if you're using Firefox, you're stuck.
One issue is that the security certificate technology does not work and play well with embedded firmware, since it has a expiration date. That is not to say this is not D-Link's problem, because it is. They need to put an updated, valid, security certificate in every firmware update. Apparently, they don't. They also apparently have allowed the security certificate for the myDlink web site to expire. They need to fix this.
But, it is also the browser supplier's problem. They need to implement a way to save expired certificates in a browser cookie or some such that the user verifies as safe. IE lets you override, but you have to do it each time. Saving the override would make subsequent logins simpler.