This question already has an answer here:
I'm having issues where my texts are sending through my iCloud. I've read the other threads in here and tried the following:
Go to Settings → Messages → Send & Receive
However, under "you can be reached by iMessage at" has my phone number greyed out and two email addresses (my icloud and my apple email).
Under "start new conversations from" it only shows my email addresses and not my phone number.
I think this is also affecting my ability to send texts via my Apple Watch.