Hey octaviantrif,
To clarify, when setting the iPhone up did you enter your Apple ID and email or sign into iCloud? Or are you prompted to enter a password for an already entered but mostly obscured email address? If the latter, this is Activation Lock and a potential sign the iPhone was previously owned.
If you did enter your own Apple ID info to login, then go to Settings and check it is your name and Apple ID at the top. If it is, then we'd recommend restoring the iPhone to factory settings as outlined here:
Bear in mind you'll need to enter your Apple ID password in order to complete these steps and erase the iPhone.
After that, try setting it up again. If you're still prompted for an existing T-Mobile account, contact Apple Support directly to take a closer look.
Take care.