How can I stop flashing amber on ATC because of "Setup over WAN" is checked?

After new modem/router supplied by ISP was installed for me ATC couldn't work with it. ISP tech rep(s) did lots of things and now modem/router (in bridge mode) and ATC work fine for all on my LAN including Time Machine backups. But, ATC has flashing amber with warning that Setup over WAN decreases network security. Can I just uncheck that box in Airport utility or are other actions needed?