RESOLVED: Again, the post asked, "It has worked for one iPad+Watch pair for a while now, why not the other pair?
As suspected, it was on the network-side. The Carrier version on the less-functional Watch/iPad pairing was "Verizon 37.0" vs. "Verizon 39.0" on the pair with desired behaviors.
I asked US Mobile tech support for a manual "push" to carrier version "Verizon 39.0", and with that, I can now send and receive iMessage and SMS/RCS messages to Apple and Android devices to/from the WiFi-only iPad's Messages App. I asked about SOC/functional codes assigned on each line, but we didn't get into the details on that before things started working.
Correlation vs. Causation? I don't know, but it's my reality.
Original Post:
I have two cellular Apple Watches, both with standalone, "watch-only" service from US Mobile. Both watches were purchased from Apple at the same time, and service with US Mobile was activated at the same time. Both are up to date on software versions.
I used an iPhone to set them up in "Family" mode, but neither watch uses the iPhone except via the "Watch" app to occasionally configure stuff or look into issues like this (and I've turned off the iPhone).
I have two WiFi-only iPads, each with using an iCloud account corresponding to their respective standalone Apple Watch.
THE ISSUE:
From one iPad, the user can send iMessage (Blue) *AND* SMS/RCS (Green) messages to Apple phones and Android phones, respectively. Most-importantly, SMS/RCS/Green messages sent from the iPad arrive on Android phones using the phone number of the associated Apple Watch. It's just like texting from an iPhone with a normal cellular plan.
iPad "Messages" App --[same Apple ID]--> Apple Watch --> Android (works)
iPad "Messages" App --[same Apple ID]--> Apple Watch --> Apple Device (works)
From the other iPad, the user can only send iMessages (Blue) to Apple devices and *NOT* SMS/RCS (Green) messages Android phones.
iPad "Messages" App --[same Apple ID]--> Apple Watch --> Android (FAIL)
iPad "Messages" App --[same Apple ID]--> Apple Watch --> Apple Device (works)
What in the world is going on???
I find references saying it should not be possible to send SMS from a WiFi iPad via an Apple Watch with a watch-only plan on US Mobile, but here we are, sending texts in the Android and Apple ecosystems just fine, but only from one iPad.
At the same time, I am not able to make the other watch + iPad combo send SMS messages (Android) the same way.
US Mobile text-based support is unable to help.
Any thoughts on running this down / troubleshooting?
US Mobile says "WiFi calling" and "MMS" are not available on watch-only plans so it's not that (or is it enabled but they can't see it?).
Unrelated? I've also been on a bunch of support calls with US Mobile over the plan lifetimes to get things like voicemail working and even just incoming voice calls working on only the one watch line that isn't supporting iPad messaging <shrug>.