Hey there "Facebook":
What prompted you to rebuild your iPhone? Did you buy individual parts from eBay and then reassemble them?
The NFC antenna is the top bezel itself, the top bezel has 2.4G(wifi and Bluetooth), cellular diversity, GPS and NFC. The main feed point for GPS, cellular and 2.4G is the screw boss under the plastic add-on antenna, with a "grounding bracket" attaching it to the bezel. The plastic antenna serves as the 5G wifi antenna.
The NFC antenna feed point is the screw boss next to the camera and sleep/wake button, with a flex cable under the camera bracket serving as signal conductor.
There is an additional feed point in the middle of the bracket, it does not connect to any wireless services, however it connects to a dynamic switch to tune the top bezel dynamically to serve different bands.
The F-shaped flex cable on the back of the board is JUST a bridging cable, not an antenna in any way.
It is possible that the grounding bracket found in step 29 of the repair guide below may be missing or broken. It isn't exactly clear when the actual "antennae" is on the 6 and higher but some people believe that it consists of the upper portion of the phone itself.
iPhone 6 Logic Board Replacement
I would appreciate hearing back from you about this.