Surprisingly enough, finding new documentation on DD-WRT, with a custom certificate, is difficult.
Yes, there is information around, however it’s date years old which doesn’t inspire a lot of confidence.

Login to your DD-WRT front end.
Go to Management -> commands.

In the commands box, paste your keys like so.
Finally at the bottom choose ‘save startup’ and your are done.
If you are concerned about it going wrong, you can run it interactively on the console first to test 🙂

Enjoy