Yes - important to get those two rectangular brackets in place first but not all the way as you say.
Re-assemble: agree most difficult process. Patience required. Used magnifying glass and wore my generic reading glasses. Found that the connector went further up to the right than was required. For what it’s worth: used 2 spudgers - one to push it back towards its cable and the other to push down.
Did the same. DONT FORGET when the loci board back in to the phone to lift all cable including especially this little antenna one!!
(I got 10 steps further along before I realised I came to this step in the reassemble and realised my mistake).
Agree with Colin Lacey above - slide the small connector from the top of the phone towards the battery ( yes, counter-intuitive) and I felt a click and was able to push down and make the connection.