Seems suspect that it doesn't come up and show a green light, can't get to the personalization steps for registration (so can't finish registration), can't connect to it on any other port than the diag tool...
I think there's something messed up with how it's powering up. If it's interrupted when attempting to dial home and/or can't complete for some reason, that's a problem that I just can't get past and there's nothing beyond it that can be done.
Just to close this one out - tried a factory reset on the advice of support with no change. They’ve determined that the device may not the hub may not have been formatted correctly and the best course of action is to replace it.
I just tried the "factory recover" for myself on one of my development hubs. It was certainly a flashback.. 
In 2018 when I got my first hub, they came with NO Platform OS installed and during the initial boot, it would go download the image.. that's exactly what this 'feature' did.
It completely erased the hub.. all files, the platform os, everything (but not the Z-Radios). After downloading a fresh Platform OS and the reboot that entails, it boots to the very familiar SoftReset Green screen with "Restore Backup" along the bottom. I did that and when it was all done, I was right back where I started.
The thing I would mention about this is that it takes a long time.. long enough you vividly imagine the Hub is bricked. For my C-5, it sits on that Blue LED on the front forever. Then I had to go to portal.hubitat.com also to 're-register' the hub, but that was barely 2 seconds, (For my first hub in 2018, that step took 'overnight' )
So.. for what it's worth, this newest technique works, but because of the time involved to download the Platform OS, does NOT replace the Soft Reset method. For those of us with more than a couple Platform upgrade experiences, we know how long it takes to do the download, verify, install, reboot. This method has one of those, but without the soothing messages 
The most likely user of this is going to be New Users who are unlucky to get a hub that sits at the BlueLED for 'hours.' But the long delay while downloading, verifying, installing is going to be a challenge to be patient through to the end. Frustration and a power cycle just perpetuates the problem I imagine. 
Maybe I’m missing your point. It seems like you’re saying it’s a worthwhile procedure but the user needs to be patient. The factory reset on mine showed a page with words that indicated that it would take a while (no biggie) however immediately it launched a pop up window that reported success. When I clicked OK, it left on the factory reset screen like nothing had happened.
If there’s was something else to try I would have. That being said, a return/replacement is 1 day away with Amazon Prime.
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