I found a Website explaining it and I followed the 2nd route B) there using my phone hotspot to connect the DTU with the internet since I didn’t want to alter my internal DNS settings in the router (running PiHole and unbound for DNS).
To get the HMS-800 back into your router, you need to turn off 5GHz Wifi first. It took me 2-3 rounds until I got a connection. After the initial succesful connection you can turn on the 5GHz again.
After successful update I recommend to restart the HMS-800 by detaching and re-attaching the DC input from your panels. I had the DTU online, but an offline error for the Microinverter in the cloud until I did the unplug procedure. After that everything was fine.
Give me a shout if you need more help on this…
Here’s the description in German:
In English via Google Translate: