I've been using Linux for at least 2 years. I have Linux Mint on my main computer and Debian on my old computer. Trying to apt update says that the connection failed to security.ubuntu.com, deb.debian.org, ftp.uk.debian.org, etc.

Updating directly from sources such as for the Brave Browser still works. Sites not necessary for updating still work. Accessing them through browser doesn't work (It would give a "Connection was reset" error (for Debian) or an error that mentioned DNS (for main computer)). Pinging them seemed to work. Accessing these sites on my phone still works (though it didn't until I accessed them through mobile data and switched back to wifi). Connecting my Linux Mint computer through mobile data to apt update and switching it back to wifi resolved the issue for my Linux Mint computer, but I wasn't able to get my Debian system to connect through my phone.

Any ideas for causes/resolutions?