invalid sim card after reset settings



    I bought iPhone 7 plus, not new, but with box and all accessories. I inserted my sim card, everything worked. made calls without problems. the firmware was 11.4.1.  a couple of days I went with the phone, bought a cover and decided to reset the settings and content, that would be I had a new phone.  after switch on I received the inscription SIM card is invalid. there is no problem with the SIM card, I tested it on other phones all works well.

    I call the Apple support, they told me update iTunes and iPhone to latest version. i did it, but the problem did not disappear. I tried to restore my phone via iTunes, but in iTunes i also received the same - "sim is invalid...."

    In Internet I came across a site on which I was given such information by IMEI:


    Locked Carrier: US T-Mobile Locked Activation Policy

    Country: Us - United States

    Simlock: Locked


    I do not understand why they tell me that the phone is locked? the previous owner went with him for a year, with his SIM card. after that I went for a few days and there were no problems. if the phone is locked, why did it work well all this time? and I'm sure if I did not do the reset, they would still be working. the seller tells me that the phone is unlocked and there should not be any problems.

    if needed, my IMEI - 353815087400655

    help me please solve this problem. Thank you!

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