I fixed my DNS issue and also had to had to fix my prosody.cfg to also listen to port 5223 which it did not by default in current version. Now I can finally use Renga and (possibly other older clients).
-- Modern XMPP client connections: STARTTLS on 5222
c2s_ports = { 5222 }
-- Legacy/direct TLS client connections: old SSL-style XMPP on 5223
c2s_direct_tls_ports = { 5223 }
By the way I also managed to login on the site using XMPP login. Took me a while the message was arriving in Renga itself (I think Monal on my phone does not support that XEP, as I got the messages but didn’t know how to confirm).
I confirm it now works both in the Depot version as well as in the debug version I build from the repo today.
Thanks a lot @PulkoMandy !