[Samsung Galaxy Watch 5] Aethera:
Thought I would post this info here as it was a little bit tricky to get the mobile plan activated with the Galaxy Wearable app on the phone. This process should work for any Android phone that has the Galaxy Wearable app installed and already connected with the watch.

When initially connecting Galaxy Wearable with the watch it will prompt to set up a mobile plan which you can skip. If you try to, you will see a screen like this:

View attachment 5791553

Don't worry, that message isn't correct. The watch can be set up with Metro.

According to a Reddit post you must have Samsung Pay(Watch Plugin) installed on the phone.

1. I recommend first installing any update(s) to the watch. Be patient with regard to updates. It's best to enable auto-update (Settings -> Software update -> Auto install) and let the watch update itself overnight (while charging). Sometimes you can expedite the update process when, after an update is downloaded and ready to install overnight, you can select the Install Now option. A new watch may have more than one update. Be patient and give it a good day or two if necessary for all updates to apply.

2. Get your IMEI and EID numbers handy. You will need to provide them to the Metro customer support rep. Be aware that because these are very long sequences of numbers, if you make any mistake in providing them to the support rep it will appear as if the watch cannot be activated or is incompatible with Metro.

3. Call Metro support at 1-888-8METRO8 and tell the automated system you want a new plan. When it lists the plans available to add, say "something else" and you'll be connected to a rep.

4. Tell the rep you're adding a BYOD Samsung watch to your plan. Provide the IMEI and when they ask for the SIM use the EID number. These are both printed on the box or available on the watch under Settings -> About watch -> Status info.

5. When the process completes the support rep will assign a new phone number for your watch and you're done with them.

6. Next you need to activate the watch's ESIM in the Galaxy Wearable app on your phone. Click the hamburger menu on the left of the Galaxy Wearable app main screen, then select the gear icon to go into settings.

7. Select "About Galaxy Wearable" and click on the "Galaxy Wearable" text quickly 5 times. This will open a special settings mode.

8. Select "Test ESIM Feature" -> "QR TestMode" and make sure that "Current QR TestMode" is ON.

9. Keep pressing the back arrow to go back to the Galaxy Wearable app main screen, then select Watch Settings -> Mobile plans. From there follow the directions to provide the activation code via QR code.

10. Use the Metro-by-TMobile QR code from this page under the Samsung section. The activation process will complete and you will see this screen:

View attachment 5791557

11. On the Galaxy Wearable app, go to Watch Settings -> Mobile Plans and turn on Metro by T-Mobile

12. After activation you'll see a prompt on the watch to install Metro-specific apps.

13. If you go back into the Galaxy Wearable special settings mode you should see that Current QR TestMode is now off.

14. If you have trouble starting the QR code authentication process in the Galaxy Wearable app, try to initiate the process from the watch via Settings -> Connections -> Mobile Networks. When it asks if the watch uses the same carrier as the phone you can say "no".

According to the rep I spoke with you can add up to five watches to a plan.