I think that's saying both the RPM Fusion repositories (free and nonfree) can't be found, possibly because the mirror you're using is down, or doesn't have F42.

Edit: When you say you did it manually, what do you mean exactly?

Check dmesg output when after the wifi drops and see what the kernel is doing. That could inform your decision. I have an old asus that started having a bunch of wifi bugs too, and I'm pretty sure they made some updates to iwlwifi. No solution though, I dont really care because that machine barely gets any use. Wifi always works perfect if I stay on a tty and don't enter a graphical session.

That being said I wouldn't choose fedora for an older relative unless they were really into computers. While it has become more stable in recent years, they do break things from time to time.

If you do decide to keep them on fedora, maybe try an atomic version. That way when things break you can just roll back with no issues and pin the working deployment. Chances are they just want a web browser and libreoffice so the learning curve wouldn't really matter to them.

https://blog.fyralabs.com/upgrade-error-with-vlc-plugins-freeworld/

That rpmfusion package looks like trouble, can you uninstall it or all of rpmfusion before upgrading? I also see internet problems here also so you might be better off fixing that first.

I'd remove rpmfusion and it's packages, upgrade, and reinstall them. No idea what's the particular issue with the 404, could be old mirrorlist, transient issues, but in general if you asked Fedora they'd tell you they only support upgrading with official repos on.

Which is fine really. You're not losing anything.

It's not resolving the mirrors. Can you open any websites in a browser on this machine? Sure seems like the network is unstable or possibly unusable.

Otherwise, just run the check for updates, install and updates that are available and see if those work.

Also, why are you specifying the release server manually?