SchildiChat for Android
Install F-Droid if you haven't already.
On old Android versions, you might need to enable the installation from "Unknown sources" in your System settings (usually under "Security").
You can find the SpiritCroc F-Droid repo here: https://s2.spiritcroc.de/fdroid/repo.
Check out this F-Droid tutorial for instructions on how to add it.
First, reload F-Droid's app information by swiping down F-Droid's overview screen.
Then, search for SchildiChat, or open this link in F-Droid. Scroll down and expand the "Versions" section. There, expand the latest versions until you see one which says "Repository: SpiritCroc's Apps", and click the "Install" button to install it.
For future updates, F-Droid will automatically use the same repository, so you don't need to worry about accidentally upgrading to a different version.