@ArielSchivo 

You can give it a go to downgrade the firmware to the previous, download the previous version from the website, unzip it, then upload it into the router:

https://www.tp-link.com/support/download/archer-c5400/v1/#Firmware

@Kevin_Z  thanks for your help but it says it cannot upgrade it, I think that's why it's a version below the installed one. 

@ArielSchivo  What's the current firmware on the C5400? If you cannot upgrade to the beta firmware we provided, can you please try to reset the router to factory defaults, reconfigure it from scratch, then monitor the 2.4G connection again?

If the 2.4G signal still disappears after some time, please try to run the Wi-Fi analyzer to check if the signal is actually broadcasting or not, try to modify the channel to different ones, channel width to 20MHz, then monitor again.