This is a follow-up on this question, which is about why it is so troublesome to add Flatpak's PPA to the repositories.
I have now added the PPA and the public key, so that I could run
sudo apt-get update successfully.
Yet, both commands
flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
flatpak install (software reference)
result in the console to wait for a long while and then output:
**error:** Can't load uri https://flathub.org/repo/flathub.flatpakrepo: Could not connect: Network is unreachable
I've been able to reproduce this on both my company's laptop and my own (both running Ubuntu 18.04).