Thank you all for trying to help, but none of these solutions seems to work for me. The large "Download Unity Hub" button still only gives me 3.0.0-beta5 , and I can't see anything resembling a taskbar in the unity hub UI before logging in.

I think I'll just use windows instead, it's less trouble.

Hello @stephanedtk, I found a fix for this.
When unity hub is opened, there is an icon on the taskbar (not sure of the word) where you can do the following :
- Sign In
-Troubleshooting
-Activate Licence
-Quit Unity Hub

Clicking the "activate licence" option will open a menu, in which you can, under "advanced" section, change the release channel. You will want to change that from "beta" to "production".
You can now, by right-clicking again the little taskbar icon, "Update and restart".

I have found a way to download the older Unity Hub 2.4.5. Go to this page https://unity3d.com/get-unity/update, set "Platform" to "Linux" and press the large "Download Unity Hub" button. This gave me access to Unity Hub 2.4.5.

Thanks for finding the workaround, unfortunately the unity developers made it impossible to find older versions of Unity Hub (especially for Linux). I could rant for hours about why it's a terrible idea, but I'm sure the devs are aware of it, but somehow still don't do anything about it.

Now I can't work because I can't sign in.

I think my only option is to change my operating system.