After some time version 07.00 is running:
I think, at least I needed the Package Download: WiFi Scanner
and in the -> Network -> Wireless section -> Global Settings I changed the default Country Code from "US" to my country, which has 2 channels more in the 2.4GHz-band. An the channel to "Auto".
Some clients are working this way, others with low power do not properly work.
And still no proper way to connect to Captive Portals, changed MAC (or DNS (not found)) but nothing.
Mainly, it is now done in the 'Wireless'-section, instead of the former WAN-section.
But in the Interface-section are some main definitions of the 'client'-mode, which I do not understand properly: wan or wwan or ?!?