Default [q] sm-n900w8 not registered on network

Hi Eveyrone,
I bought a SM-N900W8 unlocked from a guy. The phone has NOT REGISTERED ON NETWORK problem. I can see full bar but cannot make any phone call. The seller said he tried to flash cm and somehow screwed it up, so he reset and restore the phone to its original stock android. Right now im trying to make it recognize my sim. Does anyone have a solution for this? Should I root the phone and try region lock/ sim unlock?
Please let me know
Thank you

UPDATE: I still have my IMEI, its is the same as in the box