Dragino TrackerD Question - issuing AT commands

I received a Dragino TrackerD device and successfully connected it to TTN.
Currently I try to finetune the settings via serial connection to the device from my PC. I was able to connect and I see regular status messages in the putty window.
However, if I try to Issue an AT command (or any input at all), all I get is

“ERROR”

Do I miss something?
What do I need to do to successfully configure the device with the AT command set?

Dragino TrackerD Wiki
Instructions to use AT commands to configure

Thanks for any pointer.
Dan