I’ll suggest two things to try. Do the first one and test for results. If no good then do the second one.
# remove the SIM card and clean the contacts with a pencil eraser.
# Follow the guide on iFixit for replacing the antennas. I did this recently on my wife’s iPhone 6 which was having cell connection problems. It was constantly dropping calls and the phone would indicate poor signal or no service. The GPS functions were also not working well. I replaced the antennas but might not have needed to. When I was disassembling the phone I found many of the screws holding in the antenna were loose. Making sure those connections are clean and tight could be the only fix you need. In my case I went ahead and replaced the antennas since I had new ones right there. The problem is solved in our case.