Indeed, this is not normal behavior and both networks should be secured. Note that the firmware is programmed to restore all default information including the Wifi security if you perform a factory reset on the unit. This could be a solution to enable the default Wifi security and password . However, this will also set back to default any option you may have changed (eg. admin and wifi password, port-forwarding, parental control, etc.).
We sent you a private message, if you would like to send us your contact information, an agent can contact you to verify this issue in more detail. Alternately, do not hesitate to contact our technical support that will be able to help you: http://www.videotron.com/residential/contact-us.
I logged onto the router and reset the password and security options for the 5GHZ channel to match the settings for the other channel.
I also set the admin password for the router, which I had deliberately left unset. I'm wondering if one of my son's friends had 'helped' playmates log onto our network without having to type in the randomly generated password...
Anyway, if the issue reoccurs with the router admin password set, I'll contact Videotron support, as suggested.