Reolink FTP not working on X750 Spitz with Verizon 4gLTE

I have a Reolink camera connected to X750 Spitz with Verizon 4G service. The FTP function of the camera uploads a photo every 30 minutes. The FTP logs show that the connection is made but the resulting jpg file is 1kb in size (an empty file). I’m using PASV FTP mode as selected on the Reolink camera.

Troubleshooting Steps Taken:
If I connect the WAN port of the X750 router to my Comcast internet, the FTP transfer works fine. So I should blame the Verizon Service?

But if I take my same Verizon SIM card and put it into a Netgear LB1120, the FTP transfer works fine. So I should blame the X750?

If I hook up my laptop to the X750 on Verizon 4G and use Filezilla to transfer files to the same FTP server, it works fine. So I should blame the camera? I also tried multiple Reolink cameras and camera firmwares with no change in results.

If I use the FTP Test function on the camera, it successfully uploads a 1KB txt file the says “You have successfully uploaded a file…”
So it seems that there is a max data transfer of 1KB when using the X750 on 4G.

I tried setting up the email function on the camera to get around the common issues with FTP and NAT but email had the same problem - the test email goes through fine but the photo email gets stuck and the transfer fails.

This issue started on the 3.104 firmware. I upgraded to 3.203 and the issue persisted. The issue persists across 3 different X750 routers in 3 locations.

I tried changing routing / firewall / MTU settings on the X750 but nothing made a difference. There are problem some tricks I didn’t try.

What else can I try to make this work?

Very strange situation.

When you use filezilla, can you try PASV or ACTIVE mode manually? Just to make sure both mode works.

Are you able to change the ftp port 21 to others and try?