To verify if this is enabled on the server run the following:
cat /proc/sys/net/ipv4/tcp_fastopen
If this returns a value of 0, it is disabled. If this returns a 1, this means it is only enabled for outgoing client connections, and a 2 indicates it is available on the server. Setting this to 3 will enable both of those options.
echo "3" > /proc/sys/net/ipv4/tcp_fastopen
If you would like to keep this through a reboot, echo the following into a new .conf file.
echo "net.ipv4.tcp_fastopen=3" > /etc/sysctl.d/30-tcp_fastopen.conf
Ref: https://support.cpanel.net/hc/en-us/articles/360051150693-Bind-unable-to-restart-due-to-TCP-FASTOPEN