Messages to Android users are not delivered

Unless you also have an iPhone, your iPad cannot send or receive SMS or MMS messages to phone numbers - as these are Cellular Carrier features. Without an iPhone, your iPad is limited to sending/receiving messages via the Apple-proprietary Messages App.

Native iMessages are directed to Apple recipients by their registered email addresses - and not by their Cellular phone numbers.

To use SMS/MMS messaging from the iPad Messages App, your iPhone must be logged-in to iCloud with the same AppleID - and message forwarding to your iPad must be enabled on the iPhone.

https://support.apple.com/en-gb/HT208386

I hope this is helpful in clarifying your iPads Messaging features.

iPads do not send SMS.

They only have Apple's own messages, which only is available on Apple devices.

To send SMS, you have to have an iPhone and set up text forwarding.