You just missed the other thread:
I use Briar | F-Droid - Free and Open Source Android App Repository
and either Conversations | F-Droid - Free and Open Source Android App Repository
or blabber.im | F-Droid - Free and Open Source Android App Repository
mostly tending toward the former.
If you insist on a SMS type messenger that is carried over (nonfree) cellular radio network you could look at Simple SMS Messenger | F-Droid - Free and Open Source Android App Repository
though I have not tried that myself.