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[f], or open this link using the F-Droid app. Finally, click install and that's it!