Iphone 6 no service

Thanks for reading this post

I got iphone 6 from my client and its say no service
Then i disconect betrery and after service work fine and in 2 or 3 days it do agian no service
Agian i disconet bettry and it work but same think agian
I have try to flash and change charging port etc..
Any one have idea what is problm
I have seen soo many phone with this problm iphone