You will need an iPhone with cellular service
Both the iPad and the iPhone will need to be signed in on the same iCloud/apple account
The cellphone will need to be powered ON (I believe this was the primary reason my iPad wasn't able to send messages via my iPhone number or associated emails)
On both devices, make sure iMessage is toggled on. Settings > Messages > iMessage.
As far as I tested, the rest of the settings don't matter. I also tested this on my old wi-fi only iPad that hadn't been updated in 2 years and the same was true. I only noticed this texting issue on my iPads when I let the iPhone die (which was often). Your issue may be more complicated but maybe not! And I hope this helps