@Porcoline if you bought it from T-Mobile, there are 2 possible issues.
You may still have it attached to your Google Account. You can go to https://myaccount.google.com/security and sign in and see if it is still there. This may be one issue
You may also still have the phone locked on T-Mobile if you did not pay for it completely. Sometimes, people buy the phones "on-contract" under the 24 or 36 month plan and the phone is still carrier locked. If you DID NOT do this, you would still need to call T-Mobile to have the phone carrier unlocked. I had to do this for my Galaxy S8+ before I shipped mine off to trade it. T-Mobile even has an unlocking app you can use.
Hope that helps!
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