the antenna terminals(2) are located at the bottom right corner of the casing when the pcb is taken out. only one pin connected to the antenna pops out from the black enclosure. try checking to see if you screwed that part in well properly.

the second terminal is the fragile copper piece thats actually just soldered on the top of the pcb and can break when removing the pcb or replacing a screen.the copper is bent in a way to touch the aluminium/silver frame of the iphone. check to see that its still there . And if its broken replace it with thin copper wire soldered to the aluminium on the inside and the other point to the pcb.

goodluck