The first question is, does the rest of the device work? You push the power button and it does power things? Knowing that will indicate if the problem is with the battery or the display connections.
In my experience, getting those small ribbon cables for the display can be quite challenging. When I've worked on similar devices I've had to use a set of tweezers with an angled end to ensure the cable is completely inserted with the correct alignment. The cable must be properly centered, inserted all the way, and straight in. You also have to flip the latch down properly to sure the contacts in the connector make contact with the conductors in the cable. Fail to do any of that and you'll have issues.
Since you can power the device on with the case still open, I'd do that.
As an aside, removing PCBs from devices on anything other than a static electricity discharge safe workbench is very risky. I've been working on electronic devices for 45 years and I'm still as paranoid as can be.
The unit is working well it is charging, the speaker is working normal, but the screen doesn't work at all. your correct returning the those small ribbon cables for the display can be quite challenging.
I find very difficult to align it. I already give up and looking for GARMIN REPAIR center here in Japan but the repair cost the same with the unit.
