Is the PBX local or hosted? If hosted is it hosted on your instance or is it hosted by 3CX? Can you login using your public IP?
What happens if you perform an nslookup on your FQDN? Do you get the correct IP back?
We will need some more information in order to be able to assist.
Thanks for info, in the meantime, we restarted our server where we have installed 3cx, after that everything started working ok. We had some sort of ddos attack, from ip
IP 51.38.88.233 and aftrer that we couldnt login in web console at all. We couldnt open window with username/psw entry field