The updated firmware appears to have been posted (January 14, 2020 release). https://www.tp-link.com/us/support/download/archer-c5400x/#Firmware

@Carl, thanks for the response. I look forward to an update from your engineering team.

I recently purchased the C5400X and appear to have the same issue as others in this thread -- where my internet speed drops over several days and a router reboot is required to bring it back to full...