[Q] Stock modem not working

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http://forum.xda-developers.com – i just got a not my 2nd one , ok so this note was unlock by calling att so after that i put in my simple mobile card and every thing was fine till today not getting 4g or edge any thing cant make or recive calls nore msg questing is am wondering is it becase of att stock modem or do i have to root and install custom rom with a tmo modem ? i was hoping to leave phone as is just in case official update comes in on jb ... omg what do i do to be honest to what happend to my other note am afraid to flash any thing on this one but if i have to i will (HowTos)