Facetime Camera not working after Screen-Replacement

You might start out with a hard reset of the phone. For the iPhone 7 Plus this is done by holding the power button on the right hand side down while simultaneously holding the volume down button. Hold these two buttons down until the phone power cycles and starts to reboot. This by the way will not erase any personal data.

If that doesn't solve the problem, examine under a magnifying glass the FPC connector on the logic board for any bent or burnt pins - a single bent pin could effect the front camera without effecting the proximity sensor or earpiece functions. Also examine the proximity flex connector assembly for any nicks or tears.