oh well. Hopefully a fix is in the works.
Wi-Fi calling with AT&T seem only to kick in when there is no or low signal.
To simulate this:
On the phone settings -> Cellular turn on Wi-Fi calling. Then turn off Cellular data.
Wait a little bit and the phone should switch to AT&T Wi-Fi calling.
After that go to the Watch App -> Phone
Now the Wi-Fi Calling slider is visible and can be turned on.
Turn your Cellular data back on.
Hope this will work for you.