iPhone 8 Plus Lightning Connector Assembly Replacement
Check step 51, there is a thin flex cable going around the top right perimeter of the rear housing, around the rear camera position, that is the Apple Pay antenna. It needs to be screwed in to the rear housing and a screw post on the logic board.
Also all the small clips, brackets, screw holes and contact points on the rear housing may serve RF purposes, make sure they are exactly prepared and assembled like factory condition.
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