there was a development since my first post. I was able to establish a stable connection with the PUTTY telnet so I decided to do a factory reset as described in the manual(I'm frustrated after 3 days of this). As I was setting default commands after the reset, I lost connection when setting:
set wlan auth 4
set wlan hide 1
set wlan join 7
set wlan chan 8
So I rebooted the unit and after that, there was no SSID available anymore to connect to. After many attempts, I decided to open the PMC unit and see if there is something strange.
To my surprise, I found out that the WIFI daughter board was not connected properly all the way in(see picture) which was maybe the cause of the connection problems and strange behavior of the PMC unit.
So I pushed it all the way in to secure the connection but my problem is now that I still don't see the SSID and I can't connect via PUTTY.
As I understand I need to perform a hardware factory reset to get the soft AP again.
PS: here is how was the mount behaving and the LEDS after stopped responding before(see attachments).