Answer:
Ports 20 and 21
Explanation:
For the FTP to work, there are two parts that are required; the command and the data. These two parts are sent on the two ports 21 and 20 respectively. If the firewall is indeed blocking connections on ports 21 and 20, then your FTP will not work.
Hence, you need to open ports 20 and 21 to allow FTP to remain functional.
Cheers.