Using Explicit FTP over TLS

I have spent quite some time searching for a solution to this issue, but alas, I have not found one.
What I want to do seems like it should be a simple task, but I am having no success in getting it to work.

I am using Filezilla as a FTP client behind my Sophos UTM (v9.409-9 - home license) to connect to external FTP servers (for management of external websites).

When I try to use "Use explicit FTP over TLS if available" as my encryption option it authenticates to the FTP server but fails to do a directory listing.

Status:    Connection established, waiting for welcome message...
Status:    Initializing TLS...
Status:    Verifying certificate...
Status:    TLS connection established.
Status:    Logged in
Status:    Retrieving directory listing...
Command:    PWD
Response:    257 "/" is your current location
Command:    TYPE I
Response:    200 TYPE is now 8-bit binary
Command:    PASV
Response:    227 Entering Passive Mode (98,142,97,58,165,21)
Command:    MLSD
Error:    Connection timed out after 20 seconds of inactivity
Error:    Failed to retrieve directory listing

If I use "Only use plain FTP (insecure)" as my encryption option everything works fine, but that is not the way I would prefer to connect to the FTP servers.

I have viewed the firewall log while trying to connect using Explicit FTP over TLS, but I see absolutely no indication of this in the live log.
Surely I am not the only person that has this issue.

So... my question is a simple one.
How do I configure the UTM to allow me to use Explicit FTP over TLS to connect to an external FTP server when using Filezilla?