Things Indoor Gateway not connecting

Hi! I am completely new to the whole topic, and I thought it would be easy to connect a gateway but hard to program my own sensor… Turns out it was exactly the other way round.

Unpacking and googling for the installation of the Seed Indor Gateway I was trapped by an outdated example and registered the thing as a new device. After the GW staying in green fast blinking I read around the net and then followed the description of TTS / TTN. However deleting the gateway and then later claiming it did not solve the issue. When it boots, it quickly jumps over the green then red-green flashing modes into the fast green blinking and staying there for 48h without changing.

Gateway is of version:
FW Build2020-05-07 16:03:53
FW Version2.0.4
Core Version2.0.4(minihub/debug)

The TTN has seen the GW one time at around 16:45 three days ago…

How to get it to go? Thanks for your help!

Solved the problem myself bei following the really good explanation here:

Thanks to @rich-stimson for the good one!