Original post by: Minho ,
You could try to use another SIM (new or known-good) to see if that is the problem or not. Otherwise, try a hard reset and DFU restore to rule out software issues (do a backup first!). If none of that solves your problem, keep in mind that the iPhone 6 has a well documented history of "searching issues" which are addressed via micro-soldering repairs. Your symptoms are not the typical symptoms but it's something to keep in mind as well.