The 7 Plus after screen replacement shows No service.

The first thing I would try is to re-install the original screen to see if the issue persists or not. If it does, then you may have damaged something during the repair procedure. That said, the iPhone 7 series of devices have proven to be very unreliable, suffering from flex-based issues similar to the iPhone 6 Plus (Touch Disease). On the iPhone 7/7P, the fault line runs along the top of the SIM card reader and affects the Baseband CPU and Audio IC. Any short copper traces running perpendicular to the fault line while connecting to micro-BGA pads, is subject to failing.

This can happen seemingly out of nowhere and usually follows a reboot of an affected device, like when you shut it down to repair it. Once rebooted, the phone exhibits symptoms that were never present before.

If the original screen does not have the same issue, then you probably have a defective replacement screen which is causing an issue. I don’t understand how it would but nothing is impossible :>).