@mcwebster It definitely doesn't like it when you attempt a mass transfer of small files like html and CSS etc, but switching it off then allows me to transfer files with 100% reliability. As a web designer this definitely gets in the way of my workflow and I was tempted to send it back, but decided to keep it. I guess you could also decide whether it is worth switching on and off constantly or just disabling permanently depending on your usage. 

FWIW I agree we should not have to deal with problems like this and it is obviously an issue that has existed for a long time now. TP-Link should have resolved it.

@Bilko I bought a TP Link Deco M5 mesh 3-pack and had the same FTP connection issues while up/downloading many small files. I didn't realize that problem was introduced with installing the new mesh set, until my VPS provider helped to reduce the problem to my internet provider *or* router. It was hard to find the exact problem, as I had the same issues on different physical locations. Now I know the problem, I realize all other physical locations (by coincidence) have the same provider *and* a TP-Link router. So it could be some problem between TP-Link and this provider modem hardware.

Disabling the intrusion-prevention-system immediately solved the connection issues! Time for TP-Link to work this out to a firmware-upgrade, so new uses don't have to disable 

Beside this problem, I am very happy with my M5 set. Since installing it, never had wifi connection drops and perfect signal through our whole house. Thick concrete floors always caused problems with previous routers, but now all devices have a stable connection everywhere. Also the app is very easy to use, and still allows for advanced settings.

@TimNL It does seem to be a bit of a pain and since then it created problems for me in a different area. In my work as a web developer I was using an app called Scrutiny to scan websites and highlight any errors. When the intrusion system was enabled it was blocking Scrutiny whenever JS rendering was enabled and only showing a single web link, which was flagged as an error in the app. After lots of troubleshooting and tests with the very helpful app developer it looks like it was all down to this router with the intrusion prevention option enabled. When disabled the problem went away immediately...

If you are just using the router to browse the web it may be worth keeping this enabled, since it is clearly doing something, but if you need to run any kind of file transfer or scan over the internet with lots of small files, probably better to turn it off and leave it off. I agree it needs a firmware update or some kind of patch from from TP-Link. This thread is over three years old and still no fix.

@Bilko You did it! Thank you! I'm a photographer and uploading images for clients has been a nightmare since we got the Deco Mesh. Also like you, my FTP still hangs up at 50 jpeg files if I switch on intrusion prevention again as well.

I have just discovered another problem caused by this setting. It makes it hard logging in to Amazon Prime video on my TV. It was always easy before buying this router, but since then I typically have to do a dance pressing the back button and trying a few times before I can select a user profile and pass through. This was particularly frustrating today and I wondered in the it was same issue, so I went to the router settings and disabled it. When I went downstairs again it had automatically progressed to show the users that were previously blocked. 

Earlier today I was blocked by cPanel WHM from logging in with a strange message saying that my IP number had changed and guess what, it was the intrusion prevention system on this router yet again. It seems more problematic than ever since upgrading to Mac OS Monterey, which enables private relay in the settings to hides you true IP. Obviously this feature is doing something and at times it may be beneficial, but for general usage it seems to be badly configured and more trouble than it is worth.