No Internet Speed on certain download progress

Hi, recently I have issue on download any kind of files on every mobile devices. It will always become 0.00B/s speed after downloaded approximate 700MB~820MB file size. Currently I either restart the router or restart the application to continue the download progress. But for those application didn’t cache the downloaded progress, I need to re-download and stuck forever. Any idea?

Model: GL.iNet GL-MT3000
Firmware: 4.5.16