I have found a post where, with the same driver and problem, is suggested to downloaded "WICD", an alternate network manager. So I proceeded as explained on this website : https://help.ubuntu.com/community/WICD and with my great happiness :D it seems that now the driver RTL8821AE 802.11ac is working in the right way (I'm connected to wifi, without drop or lost of connection from more tha 24 Hours.