These are difficult phones to work on so I can see how this could happen. Sony phones require a special sealing procedure to prevent the screen and back cover from separating. I would recommend looking for a shop that specializes in Sony repairs or bring it back to the shop you had it repaired at. You may still have warranty through them. Ifixit has a large list of reputable shops also. As far as the cellular connectivity issue goes, there are many small contacts and antennas inside the xperia's that can be interrupted . I would recommend having a professional check all these connections and replace the main antenna board while he/she has the phone opened up.
This seems to be a big issue with S5's and Sony Experia's. Best thing to do is to make sure you thoroughly clean frame, apply a plastic adhesive activator and use high quality adhesive's. Also you need to check your frame/chassis for damage, separation, and or whether or not its bent. We've been looking for a supplier for Sony Experia OEM water proof adhesive. If anyone knows where to get it can you please let us know. Thanks