(for reference - I am having the EXACT SAME issue as found in this post: https://community.tp-link.com/us/home/forum/topic/229176)

..... Sigh I have been trying for the past 4+ months to get my router (Archer AX11000) to allow me to access my QNAP NAS from outside of my home network via FTP.  I have a QNAP NAS (TS-253A) and I was able to access my NAS via FTP with my old ASUS Router (RT-AC3200) with my current home internet service (AT&T Fiber).  I was able to do so by enabling the appropriate port service via Port Forwarding and accessing the NAS by way of the QNAP myQNAPCloud service (this seems to be a standard for QNAP NAS Servers, so this isn't anything specific for my NAS Server).

With the proper server same, port number, & username/password, I was able to use any type of file explorer to browse my NAS via FTP when outside of my home network.  Whether it was an Android-based device, iOS-based device, Windows-based device, or MacOS-based device, as long as I used the right information and had the router configured properly (I had an extensive testing session with QNAP support, so I know what needs to be set on the NAS side and router side).  I replicated the router port forwarding configuration from my ASUS RT-AC3200 on my TP-Link Archer AX11000 router and..... nothing!

I am fairly tech-savvy, so I have already gone through some fairly extensive troubleshooting between restarting my NAS, router, & switch (I have a Netgear switch that I am using for link-aggregation for my NAS device as well as for plugging in devices that I do not want connected via wireless (such as my VOIP adapter, my main desktop computer, etc.); power-cycling all of the above devices (along with the internet Gateway and fiber translation device (for those that have AT&T Fiber - you know what I am talking about 😁)), using different FTP port settings for both the QNAP NAS & port forwarding on the AX11000.  *for those that may be wondering, my ASUS router did not have any LAG supported ports (hence the Netgear switch) and it ended up being best to leave it (the switch) integrated into my network to continue to be able to access my NAS (TRUST ME, I tried to remove it and use the LAG ports on the TP-Link..... yeahhhhh, let's just say "if it ain't broke, don't fix it!!")

No matter what I do, I have yet to be able to connect to my NAS via FTP when outside of my home network.  Inside of my home network, I use SMB to browse the NAS, so FTP is not needed internally (for those that are wondering).

I have 13+ screenshots of the various configuration settings from my NAS, AX11000; error messages from the chosen file explorer on my mobile devices, etc. to somewhat prove the troubleshooting I have gone through.

As I referenced above (& confirmed via the separate TP-Link community post), THIS WAS NOT A PROBLEM WITH MY ASUS ROUTER once I configured everything properly!  In the above post - there doesn't seem to be a resolution that was stated, so I thought I would post my problem to see if there's been some kind of remedy from TP-Link that they have not yet published to the public.

Thank you all and I hope this will be helpful for the community once a remedy is discovered (that does not require for me to use an entirely different router (I am close to investing in one TRUST ME!)) 😁