iPhone 6 can't find network
There are two possibilities.


Broken SIM card. This would be more likely because SIM cards are small and break easily. However, you mentioned that the SIM card works ok in other phones. I would still suggest getting a replacement SIM card, as that will be easier than the other possibility.
Problematic phone. It is possible that the problem is with the phone itself. It could be a hardware problem, a firmware problem, or a software problem. The only way to tell would be to take the phone to an Apple Store (or, if that's not possible, an Apple Authorized Service Provider) to get it checked out.