Hi Dirk,
thanks! That helped a lot.
I did not have a main check at all, so renaming the key in check_info to ‘ttn_gateway’ solved it => the parse function gets called correctly.
Thank you also for the suggestion about removing the for loop in the check function. Now a feel a bit stupid 
One last thing: the info that is passed into the parse function is still preprocessed. So I had to change the code of the parse function a bit, because I actually do not get the raw output of the agent (other than I expected), see Reworked version of check_mk check ttn_gateway · GitHub for the current version.
I used the join method to restore the lines – see line 26.
Is there a better way of doing that?
Markus