Why "No Service" after replacing LCD screen?

I just replaced a broken screen on my iPhone 6, and now the phone says, "No Service." It detects the SIM card, and I was able to go to the home screen after inserting the SIM card and connecting to iTunes. But it still does not detect any service networks. It says, "Searching...." then goes to "No Service."

There might be a few screws that aren't exactly the right size, and I've tried switching some of them to no avail. I am sure that the screw for the battery connector is correct. I am told the antenna is connected via screws. Is this correct? Any help would be appreciated. Thanks!