WiFi connection is unstable when uploading (or perhaps downloading) large files with FTP

I use an app like WiFi FTP Server or Files on Xiaomi Poco 3. Everything works perfectly. However, often the upload break. I notice that I cannot access the internet either when that happens. Quickly turning on and off WiFi fixes the issue and if fast enough, the file transfer will resume. I use an app for pinging. . As you see, after a while, pinging the router IP will fail. However, turning off and turning on WiFi quickly will restore connections. This is of course a very annoying problem. That's because I have to keep guarding the smartphone. As far as I know, this is only happening when uploading large files. If I do nothing, the WiFi is fine. I am using WiFi FTP Server app on Android and FileZilla on my computer. I don't think the problem is in either software. Using Xiaomi's own Files will have the same problem. How to fix this? What can possibly go wrong?