The problem may be that Hetzner is blocking ports 4081–4085 at the network level or in an additional security system beyond the operating system firewall. You should:
Check the firewall from the Hetzner Cloud Console or Robot Panel (if used).
Check if Hetzner has applied additional ACLs or network filters.
Test connections to ports 4081–4085 on the Hetzner IP using telnet or nc from the WHMCS server.
If the ports are still blocked with the firewall open, contact Hetzner support directly to confirm that there is no port blocking policy on their side.