DIAGNOSIS This is usually the router's "Smart Connect" feature trying to be too clever and bouncing your device between the 2.4GHz and 5GHz bands, which it presents as a single network. The drop is it switching you.
FIX Log into the router's admin page (tplinkwifi.net). Go to Advanced > Wireless > Wireless Settings. Find the "Smart Connect" option and disable it. This will split your network into two separate SSIDs, usually ending with "_2G" and "_5G". Connect your main devices directly to the 5GHz one for stability.
IF THAT DIDN'T WORK The router might be overheating. Feel the top. If it's hot to the touch, move it to a more open area, away from other electronics, and make sure the vents aren't blocked. These things can throttle performance when they get too warm.
HEADS UP If you disable Smart Connect, you'll have to reconnect all your devices to the new network name. Write down the new passwords first—they're the same as your old one, but your phone won't know that automatically.