Cannot receive iMessages on iPad

When I go into my Apple ID account through appleid.com my cell phone number (in addition to my email address) is listed under "Reachable at". My iPad, cell phone and Mac are all listed in the account. If I try and use my iPad to send/receive messages, I can only do so using my email address. I want to use my cell phone number. But I can't get my cell phone number to appear in my settings/messages/send and receive.

I am running iOS 11.4 on my cell phone and 9.3.5 on my iPad Mini. Both have the most recent updates. A few days after I installed the 11.4 version on my cell phone, I did receive a notification if I wanted to use my cell phone number for iMessages. I received the same notification on my iPad. So it's working now; however, every few days I receive the same notification and need to "accept".