I may sound a little silly but I finally got the wireless charging pad installed on my second try.
it seems that the plug on the NFC pad got no locking mech, many thanks to
Fpangalos for the info!
If anyone feels it's hard to remove that final plug (it is for me), a screwdriver may help a lot as you could pry the plug up by levering the edge with it. Holding your finger at the other side of the plug could prevent uneven force being applied to 2 sides of the plug and making it easier to come out.
Like this: