I tried to upgrade from Ubuntu 18.04 to 19.04, but every time I tried running
sudo do-release upgrade, it throws an error: "Failed to connect to https://changelogs.ubuntu.com/meta-release. Check your Internet connection or proxy settings". Pinging the given site said: ping: https://changelogs.ubuntu.com/meta-release: Name or service not known
However, I am able to access the changelogs site, as well as using wget to get the contents of it. Apt update also returns no problem. I have followed the answers from here: failed to connect to http://changelogs.ubuntu.com/meta-release, as well as here: Failed to connect to https://changelogs.ubuntu.com/meta-release-development. Check your Internet connection or proxy settings, but none works for me. I have also changed the proxy settings from 'manual' to 'automatic', but it doesn't work either.
For reference, I upgraded from 17.04 to 18.04 by following the following: https://www.linuxbabe.com/ubuntu/upgrade-ubuntu-16-10-17-04-to-ubuntu-18-04