Thanks for the offer.
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.