Welcome to the forums. I don't have any Apple devices myself, but since you disconnected your old iPhone from iMessage, perhaps you can reconnect to your Apple account with the iPad? That may get everything back up and running.
As I was typing the above suggestion, I thought of another idea: If a phone number is required (wouldn't really surprise me) get a prepaid SIM for your old iPhone, and reconnect to iMessage. That should then work with the iPad.
Try the first idea. That way, you aren't spending any extra money right off the bat.
You may also want to consider asking on our sister site, iMore:
http://forums.imore.com/ask-question/