Upgrade from v14 to v15 went quite smoothly but after installing the new Windows 3CX client we don't have any presence on LAN or remote 3CX clients.
No changes were made to the firewall.
Make sure that no antivirus software is installed on the server and windows firewall is off. For remote extensions make sure that port 5001 is open on the firewall.
Despite having every 3CX path excluded in BitDefender including disabling the firewall module and Windows firewall - uninstalling BitDefender resolved the issue.