You cannot message with an Android phone from an iPad using the Messages app. It will not work, it has never worked and it is never going to work because Messages is an iOS/Mac only app. Phones can text with other phone by using SMS messages which is totally different than the iOS only messages app. Messages will work over WiFi, and SMS messaging will not.
You can add your iPhone number back to the iPad as the"send and receive from" caller ID and when you do that, your messages will sync across the iPad and your iPhone again. You will probably have to sign out of Messages on the phone and the iPad and then sign in again on the phone first, and then sign in again of the iPad. The phone number should be available to use in Settings>Messages>Send and Receive>You can be reached by iMessage at.
This may be of some help.
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
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iOS: About Messages
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To send messages to non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8
How to Send SMS from iPad
You can check the status of the FaceTime/iMessage servers at this link.