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.

"Congratulations your Ubuntu Linux kernel now has working Realtek wireless drivers."