What to do after a fail firmware upgrade ?

You have to think from their point of view. Their time is money. You modified your NAS (no matter how little) and a world of scenarios opens. Their policy is to say "we can help you only if you don't tinker with our devices". What they may do is to offer some sort of paid support for your cases, I would agree on that.

If I connect to my VW main computer and VW realises it, my warranty would be immediately invalidated. If I changed ONE parameter on the on-board computer which caused a malfunction, the only official solution from VW would be to replace the whole thing and that would cost you buckets of money. If you're looking for someone to access the thing again and fix the bad parameter, that wouldn't be VW, you can rest assured of that!

I know you just made a small change to your NAS and possibly that's not what caused the problem but to be honest I understand Netgear's reply. Sure, they could have advised you with a few basic things as mentioned.