How did your customer buy a Secure Certificate if DirectAdmin didn't allow him into the Certificate configuration page because he doesn't have his own IP#?
Since it's your server you need to get another IP# from your provider if you don't have any free IP#s available.
Then you need to set up the IP# on DirectAdmin for the reseller account owning your customer's account, and then assign it to your customer account.
Then either you or he can get a CSR and order a Cert, and then install it.
Jeff
Then you