Hi,
as I wrote in my post, I LOOSE connection after some minutes. This means that it STAYS connected to my wifi network for a while. This also means that the connection has been properly established (SSID, password, ...).
I've contacted AirTies directly and they are trying to replicate this problem in their lab.
Anyway, my infrastructure allows to use the AirTies Air 4400 as access point and, by configuring the device as access point , the problem disappeared.
Ralph