In order for you iPad to be able to send and receive texts, it needs to be assigned the SAME AppleID as the iPhone. If they are on the same AppleID, then you need to go and turn it on for your phone to forward the messages with the iPad.
To do this, ON YOUR PHONE, go to Settings > Messages > Text Message Forwarding. Here you will see the list of available devices to forward the messages to. Make sure it is turned on, once you turn it on, you should start getting messages on your iPad as well when it is connected to WiFi, or if it is cellular enabled and has a plan then you’ll get them when it has cellular connectivity as well.