add second phone number to imessage

My old phone was damaged and Apple asked me to turn off "find my phone." (I don't know whether that is relevant.) I decided to get a new phone, and number, during the repair process. On my new iPhone, both numbers both numbers are registered under my ID. But my iPad mini 3 stopped accepting texts from the old number. On the setting > messages > iMessage screen it lists only my new number (and 2 email addresses. How do I get my old phone number back? I didn't intentionally delete it. Is there a limit of 3? I can remove the checkmark from an email address, but I don't see how to add the number.

Thanks for any help