Sounds like you got an additional firewall between - as you already debugged, the port is present with netstat, also the custom panel port will be accepted from iptables. I can’t find any reason, why it should not work.
I have only one VPS with Ionos that I use as secondary DNS. The firewall didn’t let me in and took me several attempts to open it. In the end I deactivated the Ionos firewall.
By the way @chupa, if you don’t know, maybe you should look up what does “chupa” mean in Spanish.