The flatpak has been working for me on Linux Mint, although the flatpak didn’t work on Ubuntu proper. From what I understand the Mint devs do some extra tinkering to support flatpaks better than vanilla Ubuntu. Have you tried downloading the binary and running it from a terminal?
The Appimage also works well for me on my Ubuntu XFCE box.
Fairphone 4 with eOS, escaping Google and Apple’s duopoly is quite liberating and not as hard as I thought it would be. Yeah, eOS is technically Android, but deGoogled.
https://e.foundation/