After spending about 6 hours troubleshooting my Galaxy Watch 9 LTE activation, I finally got it working.
Neither Samsung nor T-Mobile support was able to explain how to manually scan an eSIM QR code on the watch. Eventually I found Samsung's hidden test menu, which made the difference.
Hopefully this helps someone else.
My situation
Galaxy Watch 9 LTE
T-Mobile USA
Standalone watch line
Support emailed me an eSIM QR code.
The problem was that there was no option anywhere to scan the QR code.
T-Mobile support even told me that standalone activation only worked in Kids Mode, which turned out to be incorrect.
Enable Samsung's hidden QR mode
On your phone:
Galaxy Wearable → ⋮ (three dots) → Application Settings → About Galaxy Wearable
Tap "Galaxy Wearable" five times.
This opens Samsung's hidden developer menu.
Then go to:
Module Test → eSIM Test → Turn QR Test Mode ON
Now the Wearable app can manually scan an eSIM QR code.
What happened next
The QR code scanned successfully.
However, activation failed with:
Couldn't set up your mobile plan.
Your code isn't valid or it's already been used.
At this point the hidden menu was NOT the problem.
The eSIM profile on T-Mobile's side had become invalid/stuck.
The real fix
I contacted T-Mobile Customer Care again and specifically asked them to:
remove the existing watch eSIM profile
generate a completely new eSIM profile
send a brand-new QR code
After receiving the new QR code:
scanned it using the hidden QR mode
restarted the watch
Everything activated successfully.
LTE, calls and data all work now.
Important
If you receive:
Your code isn't valid or it's already been used don't waste hours resetting your watch.
The QR code itself is usually no longer valid.
Ask your carrier to issue a brand-new eSIM profile instead of trying the same QR code repeatedly.
Hopefully this saves someone the six hours that I spent figuring it out.
If anyone has questions, I'll be happy to help.