Wierd Network issue with iPhone 6!

Hello everyone ! I work part time as iPhone repair technician and usually replaces screens and other parts of iPhones or iPads. I recently replaced a motherboard on 6 now phone's network is behaving weird it works for hours when using it no issues at all but sometimes when it's idle it get's no service thing. If i take battery out( Airplane mode or taking sim out results same no service) and put back n start it then it gets to searching and shows searching plus LTE sign i'm able to use internet as well make calls but searching keeps coming along 3-4 minutes even in carrier in settings it shows network name. Now if turn airplane mode on or off it even shows carrier name at top !! What it can be ? Can replacing the antenna come lighting part might solve it ? Any suggestions welcomes. I have followed guide as well no part or screw is missing.