Captive Network Assistant automatic reconnection

When I am away I am connecting to to a Vodafone Community hot spot with my Vodafone username and password. Which don't change. At the beginning I was connecting via Safari then one day the Captive Network Assistant (CNA) window showed up. I thought it was annoying because it didn't keep the username/password while they are automatically filled in Safari. So I used to close it (quite few times on occasions) and login via web browser as usual. Then one day I logged via CNA and I noticed that when the connection went down it automatically reconnected. Meaning it did store this time username/password. Now they are cleared again.

Since that is a cool feature is there any way to make CNA permanently remember username/password? Yes, i did flag the "remember me" box, but as I wrote it is random. Any cache to be deleted, or plist file to configure? I couldn't find anything. Thanks.

PS: I am running Yosemite 10.10.5.