Solved the problem! 🕵 😎 😇
Thanks to @benco in his post Viper TQS Calibration procedure
I was able to fix the DX 18 spot with the calibration after a couple of tries.
The magic trick to get the calibration to work is -
either to
keep button 5 pressed WHILE pluggin in
or simply press button 5
longer than 3 seconds.
Either way, you get no visible feedback that the calibration is in process, so you always have to guess and try.
See you in the sky, folks 😁