No tethering after amber update

[breakpads]

Hello I have an unlocked nokia 920 att phone on T mobile service. After the amber update i can not tether to my lap top. For some reason there is a problem it keeps telling me to upgrade to the service witch I have already. it worked fine prior to upgrade. I have checked the Sim card on other windows phones, and it works fine it must be a problem with my phone. and t mobile can not figure it out they gave up on me but have no problem charging me for the service lol I think it is a conflict with att phone and t mobile any idea's