@Azrael 

One of the users mentioned seeing the issue after updating to the latest firmware. You can try either downgrading the firmware to see if that helps, uploading the latest firmware it already has to see if that changes anything as well. 

If you still have the same issue then the router may be malfunctioned since all the other basic troubleshooting steps have been tried.

@Tony I've already tried that. I was on the live chat with one of the TP-link customer service reps yesterday, and was even given the beta version of 1.1.4 to flash and try.
Nothing i do is resolving this issue. I tried two more factory resets this morning as well. Still the same problem.

Wired ethernet works fine, but all wifi connections is dead after a few feet away from the router.

I only bought this router in February. I'd like to request for an advanced RMA.

@Azrael 

Please contact our support team, you mentioned you spoke with live chat, follow up with them again as they can begin the replacement process.

@Azrael The fix for this is done by setting wifi transmission power to low, saving config, then setting back to high, saving config.

it's ridiculous but it works, I have 3 of the archer c5400x's and had to do this on each of them after upgrading the firmware.

will never be purchasing another tp-link product. especially after one of the routers literally factory reset itself the other day while I was streaming netflix... unreal.