help i have put so much money into this phone i need it to work

Hey guys was just wondering if i can get my phone to work plz help me out...

i have recently purchased an iPhone 4 IOS 6.1.3 from a local iphone repair shop here in my home town.. when i left after buying the phone i was able to use my home screen and i could go to my setting>about>where it stated the phone carrier was ATT.. i was told i could unlock the phone thru ATT where i could use it with any other carrier.. When i contacted ATT rep they had me connect thru iTunes and restore and update the phone .. For a long while i was stuck on the activation screen and couldnt get passed to see the home screen... I just recently had the idea to put in an activated sim from an ATT customer into my phone to get me passed that activation screen and was successful.. i also have completely reset all setting on phone including the network settings to where now instead of it saying Carrier>ATT it showed Carrier>NONE.. so i thought my phone was free of the ATT networks and was able to use my T-Mobile SIM to get on the T-Mobile Networks.. now it just says NO SERVICE.. then with me still being able to access my home screen i went back into setting>general>reset i went ahead and reset and erase iPhone now .. i am back to not getting passed activation screen.. its showin insert sim and i put my tmobile sim inside the phone and it tells me its not correct for the carrier and its not supported.. and i cannot connect to itunes to restore nor can i see my home screen ..

Now with me explaining that to sum it up if it showed no carrier why is it my Tmobile sim will not work ... and how can i get it to work