TryHackMe Reverse Shell Issue: SYN Received but No TCP Handshake

The user reports a reverse shell issue on TryHackMe where PHP code execution works, but reverse shells fail to connect. A listener (nc -lvnp 4444) is active, and tcpdump/Wireshark shows SYN packets from the target to the listener port, but no SYN-ACK or completed TCP handshake. The PHP reverse shell times out with a 504 Gateway Timeout, and fsockopen() returns "no socket." Multiple reverse shell variants exhibit the same behavior.