This clip on the rear of the unit is particularly tricky to pop off, but popped off when the other chassis clips were all removed and held open, while pulling it backwards.

There are 3 clips down each vertical side, 2 on the top middle and 2 on the bottom middle.

This is where the screws screw in to. They need to be unhooked from the base to be able to pull it backwards and off.

Getting the chassis off was particularly tricky, I recommend borrowing a second pair of hands if you can.