For those just starting out & needing to Know…Open the client, select “manage repositories” from the overflow menu and then add the repo URL. Just opening the url on a device that has F-Droid installed should work in most cases as well. The F-Droid project is not responsible for any third party repositories,
So where do you find these? This is a list of repositories that are compatible with the F-Droid client application (*NOTE it’s like a Starter-List for those just starting out) : Known Repositories
Now if you happen to follow those & STILL GET a red message about the Repo being “UNSIGNED” so far the more experienced say to Uninstall the repo & reinstall. TRY the repo 1st before doing the entire app!!
Doing a Update by refreshing through pulling down from the top is supposed to fix these issues. Android 8.0 with F-Droid 1.7.1 installed has not yet resolved this issue on my Android tv box, but there are other’s who have had it work…
Gitlab & the News tab from the Main site can give more links to more advanced repositories if your in search