FTP server down most of the week

We have a WS-FTP server onsite behind a firewall/router to recieve client files from overseas. This system has been in place for 5 years with no problem until this week when clients reported the inability to connect. Testing showed that we could connect via port 21 to the server, but opening the data link for pulling a directory or files would cause whatever client software to hang.

Efforts to resolve:
1. We first verified that we could connect with no problem to the FTP with a LAN connection demonstrating that the server was functioning as expected.
2. We next removed the firewall/router and reconfigured the Comcast modem to forward port 21 traffic to the destination server. We could still connect successfully on the LAN, but WAN connections continued to fail.

3. We then reconfigured the server with a public IP address and connected it directly to the Comcast modem, WAN connections continued to fail.

4. Considering the possiblity that WS-FTP or Windows 7 Pro might be at fault, we downloaded Filezilla FTP server and installed it on a different machine (Windows 10 Pro). We then reconfigured the Comcast modem to forward traffic to the new server and tested. Again we experienced the exact same failure.

5. After testing the connection with two different FTP servers on two physically different hardware platforms with no routers, we suspected a problem with the Comcast modem. We contacted Comcast and described the issue. Several resets were attempted on the modem, none meeting with success.

6. Since the resets did not resolve the issue, we requested a new modem. Comcast replaced the modem, we configured it for out network and tested, but the problem persisted. The local support team advised us we would need to call into the main help line and opened a new ticket requesting tier 2 support. We followed their instructions and opened a new ticket. After 24 hours we recieved no response and called back in. We were informed that our ticket had been resolved and closed. For some reason they could not explain why the ticket had been closed. We opened a new ticket 22 hours ago and are awaiting contact.

7. In the meantime we performed some more testing. In our latest test we configured two firewall/routers (Netgear and Linksys respectively) with two public IP addresses from our pool of five. In this configuration, we established an FTP connection and successfully tested demonstrating that the connection passed from the client to one router, into the Comcast modem, on to the second router to arrive at the server. The connection worked perfectly as it does on the LAN. Excited that we may have found the resolution since we could transverse public IP's successfully, we attempted a connection from a WAN connection only to discover the problem still existing from any WAN connection.

If anyone has any other suggestions, we'll be glad to test, however based on the tests we've performed so far, it seems pretty apparent there is something up at the WAN level beyond our control and really need Comcast to address this issue ASAP.

Thank in advance.