How To Disassemble the LEGO Boost Hub?
Here's a YT vid on the disassembly, but the still picture shots with no commentary may not be what you need for a good walk-thru.  

I don't have a Hub, but wish we all had a pic to make sure we're talking about the same piece.

Regardless, I'm gonna take a crazy shot at it.  Let's Go!

We have two gray rectangles:


Big Gray Rectangle on the Left.
Small Gray Rectangle on the Right.


I think you're stuck here. (See the picture).



So, you took out the two hex screws, (two empty black holes on the Big Gray Rectangle), and you say you can't get further.  Correct?

Instead of trying to remove the Big Gray Rectangle plate, first remove the Phillips screw in the middle holding the Small Gray Rectangle down.

Look at the next picture.



See how the Small Rectangle has a lip design that overlaps, and locks down our Big Gray Recky-reck?

You should be good from here.

EDIT+++++++

Real issue: White Tabs won't release.


Look at this next image of the Big Gray Rectangle removed below.


 


Focus your attention to the white tabs.
Do you see the notches on the tabs?
The inverse of those notches are on the inside lip of the Big Gray
Rectangle.
When all notches are together it forms a Mechanical Lock.
We have to disengage both tabs together.
Position the entire piece like the picture below.
Get two flat head screw drivers.
Put both screw driver heads on the LEFT side of both tabs.
Hold both screwdrivers in one fist like Wolverine's daughter.
Now pivot to the LEFT which should separate the physical locking of
the two pieces.
You might need your other hand to lift Big Gray    Rectangle  up
and off.