Stereo help - I'm pulling my hair out!!

[mafty]

I have a 2019 custom which had the old 4" screen with the 6 buttons.
I am trying to upgrade to sync 2.5 (I don't need Nav as my phone has it and I don't want the hassle of fitting the APIM unit).

I have bought a head unit, model number LK2T-18D804-HH and the corresponding screen, but when I connect it, although the buttons light up, nothing else works.
It will not power on and I can get no further.

Can ANYONE please help me with this?
What am I doing wrong?