mobile

I've read through multiple threads on this site and many others to try to find a solution to my problem. I have a n900t stock rom, carrier unlocked that i have been using with straight talk/AT&T service since december of 2013. Well as of a month ago I updated my apps as i always did per the phones notifications and my signal started to just disappear. It has gotten to the point that the signal has permanently gone as i would get the message everytime i go to place a call "Not registered on Network". So i was told by the local phone repair techs to update my phone to the lastest firmwire, which took me from 4.3 to 5.0 and still no signal at all. It's not my sim card as i have a backup at&t phone which i currently have the sim card in now getting a strong signal but my note 3 is without a signal with the peace symbol in its place. My baseband is NB4 and my Imei is still there as i don't have a null imei and if it helps when I dial *#0011# it yields:
No Service
NETWORK: UNBLOCK
IMEI Certi: PASS
Can you please help me and tell me what's going on and how i can get my phone to function again?!?!? thank you in advance!!