Most likely ufw, but you shouldn’t do this. MySQL sends data and passwords plaintext. Use a SSH tunnel.

Its True that MYSQL sends data in plain text format, but with that said do the following to check if your server is reachable

• Ping the Server from the client node
• use NMAP or telnet to see if you can telnet to port 3306 [Windows Use : Dtelnet, Linux Already has telnet]
• Also see if you have firewall enabled on the server.