I didn't buy this used iPhone but my mom did, she doesn't know much about iPhones but she just wanted to give me a new iPhone, the iPhone model is the 7+, I tried to put my at&t sim card into the but it showed "Sim Not Supported" but it still showed signal and the LTE also worked. So I checked the imei and it was blacklisted but when I checked for at&t it says that it is clean on at&t but reported stolen/lost on T-mobile and Sprint. In the settings of the iPhone it says it belongs to at&t, I have no idea what to do. Please help out, Thanks
EDIT:
IMEI: 356694082743758
EDIT:
IMEI: 356694082743758
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