@phaedrux
Thank you for your support I could solve the problem.
I just used the comand M587 as it worked with my old DUET2wifi Firmware 2.. with intial ssid and network key with slash as seperator.
With the new Firmware (updated now no to 3.4.0) I have to use the user assigned network name and key without seperators.
Instead of
M587 S"FRITZ!Box 7530 DL" P"1234/1234/1234/1234"
M587 S"StefanHeim" P"1234123412341234"
Regards,
Stefan