Sorry that you're having an issue here, but it is super odd that this is happening. Basically you've confirmed that you've received the unlock for DIGITS for the device by putting it into the Note 5 and that it popups the settings. Doing the same thing should happen in the S8.
To confirm it is a T-Mobile purchased S8?
Also I'm assuming that you've probably rebooted the device multiple times?
If neither of those work, what I'd suggest is the following:
1. Make sure you're logged in with your Samsung Account
2. Force it to do a full back up of your device
3. [Factory reset it] really don't want to recommend doing that, but luckily the Samsung back up for the device is quite good
4. See if that unlocks the multi-line settings
If that doesn't work, please let us know since it indicates that there must be some other issue with either the firmware or some configuration change that hasn't been otherwise accounted for.
Thanks for helping out!
I can confirm a full factory reset of the Galaxy S8 enabled Multi Line immediately! Thank You
Evan, when I spoke to support last week I was told Multi line is still not functional with Samsung. Has that changed? It seems unreliable on my S8+