How can I get my phone back to being "network lock" ?

[AC Question]

I unlocked my Samsung Galaxy s4 with a code purchased online and a few weeks after it started giving me problems with detecting the sim card and even with the calls, is there any way I could "undo" the unlocking? tried factory reset and still unlocked.

 

[cellco]

unlocking by code will not affect the phone in any way except for ability to accept non original sim. it's the fact that you are using it on another network can create abnormalities.