Surprisingly enough, finding new documentation on DD-WRT, with a custom certificate, is difficult.
Yes, there is information around, however it\u2019s date years old which doesn\u2019t 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 \u2018save startup\u2019 and your are done.
If you are concerned about it going wrong, you can run it interactively on the console first to test \ud83d\ude42

Enjoy

#!/bin/sh 

cat <<EOF > /tmp/key.pem
-----BEGIN RSA PRIVATE KEY-----
## YOUR PRIVATE KEY HERE ##
-----END RSA PRIVATE KEY-----
EOF

cat <<EOF > /tmp/cert.pem
-----BEGIN CERTIFICATE-----
## YOUR CERTIFICATE HERE ##
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
## YOUR INTERMEDIATE CERTIFICATE HERE ##
-----END CERTIFICATE-----
EOF

mount -o bind /tmp/cert.pem /etc/cert.pem 
mount -o bind /tmp/key.pem /etc/key.pem 

stopservice httpd 
startservice httpd