Under the iPhone’s battery plate, there is a silver connector which has to be unscrewed prior to disassembling the iPhone. If this connector has not been replaced properly, the iPhone will fail to connect to a mobile network. Check the post below to deal with the problem:
[http://www.hcqs.us/blog/fix-iphone-no-service-after-screen-replacemment|How to Fix iPhone No Service after Screen Replacement]