Creating and sftp server in Mojave

I'm trying to set up an sftp server so that an associate can push media files directly to me. The server is definitely up and running since I can access it on my local network; however, when I try to access it from outside of my network I get a failure to connect. We normally use Filezilla for access, but I've tried other clients as well and none of them work. So, here's what I've done so far:

1) In System Preferences/Sharing - Remote Login is turned on.

2) In System Preferences/Security & Privacy/Firewall/Firewall Options - Remote Login (SSH) indicates active.

3) In Terminal, entering "sftp localhost" returns an "sftp>" prompt which indicates the server is active.

4) In Filezilla, if I enter the local iMac's network address (i.e. 192.168.0.15) I can connect to the server.

So far, this all leads me to believe that the sftp server is running properly. The problem is when I try to access from outside of my local network. I'm using DuckDNS to handle dynamic routing for the URL being used to access my iMac. In fact, that is working properly. (dDNS Broker is the app I'm running on the iMac that talks to DuckDNS to set addressing. See below.) My current external address is 184.98.162.86 and the iMac is sitting on 192.168.0.15 as shown below:

So here's where it gets weird. Filezilla indicates that it's connected to the server, yet it times out. Perhaps I'm misinterpreting this?

Running the netstat command in Terminal shows that a connection was made (based on the SYN_SENT entry):

Yet the sftp client times out without a connection. I've also tried bypassing duckdns by using the external IP for the DSL cable modem (184.98.162.86) but I get the same result.

And yes, I have set up port forwarding on the DSL modem. I know that port forwarding is working because I've got another device that is working properly from outside of the network. Those ports are the first two in the list below.

I'm stumped and out of ideas. I assume that the sftp server keeps a log but I haven't been able to find it. I was hoping that probing through the log would provide some information. This truly doesn't make any sense. I'm hoping that someone here has this working and can provide some insight.

Thanks.