what is wrong with my phone? how do i make it work please help?

So I live in Mexico and my friend from USA gave me their phone (samsung s6) with sim card and all. After he got back to the states he bought a new phone he got a new sim card and so obviously the one in my phone stopped working, so i went to some shop here and they said to give them three days to make it work because my sim card i bought is a Telcel one and my friend was with AT&T. anyways after getting my phone back i noticed there is a circle with a line through it instead of signal bars and I checked the do not disturb feature and that was off. I tried making a call but it tells me mobile network not available.. basically the guy from shop said he couldn't get it to work... I think he may have rooted my phone because i downloaded root checker through wifi on play store and it says " root access is not properly installed in this device" anyways i did a hard reset and it still says that and sometimes when i mess with the settings it gives me an error message saying "unauthorized actions have been detected restart your phone to undo any unauthorized actions"... I thought maybe it was a carrier unlock issue but now i fear its something worse because I checked my phone and the imei said unknown...how do i even begin fixing this i only have access to a computer and wifi what do i do to fix?