Well it sounds like you have checked everything I can think of. I know firmware download issues, at least in the software I design has to do with MD5 mismatch errors. We use a multithreaded downloaded that reassembles the firmware parts into one large file and then verifies each individual file against its MD5 hash to check the integrity before it assembles. Our process will hang right before it copies (as it's assembling) if the files do not match. But your issue now seems to be like a craft communication issue still of sorts. Multiple computers without avs and different internet connections at this point seems to have exhausted all options. But what's even more odd is that you can't update via the application on a mobile device either.
I really wish we could download the firmware files and flash them directly through the DJI app, offline.
I have the
P4P and not the Plus so I can't replicate your setup exactly but I'd think being Plus would have nothing to do with it because the AC firmware is pretty similar if not identical right?
How long have you had it? Have you ever been able to successfully update? Are you able to fly at all or are you blocked by the required firmware update?
Jesse