I can’t install anything because connection to debian.map.fastlydns.net is stuck at 0%. However, pinging it does work. I suppose it is caused by my ISP since. So, what should I do to fix that? Is switching to a mirror going to help?
if you’re having trouble with fastly, which is what hosts the main repos (deb.debian.org), then yea, try a mirror.
Is it resolving to an IPv6 address? If so and you don’t have fully working IPv6, try disabling IPv6 on Debian. I’m mentioning this as a possibility because I’ve personally had this issue with some new Debian installs a few times. The first few it took me more time than I’d like to admit to figure out the issue. If this is the case, then an IPv4 only mirror would avoid it, but you may have other random issues.
If it’s not related to IPv6, then I’d just pick different mirrors until you find one that works. Are you able to access that fastly mirror from another device (via http)?