Do yourself a favor and don't send it back. Go somewhere else you can connect to WiFi and see if your phone holds a stable connection there.
I have this EXACT same problem on my One M8 with my work wifi. Ever 60 seconds, disconnect, reconnect. Otherwise works fine. I've tried RMAing phone for a new one, tried a custom rom at one point. Nothing worked. It works perfectly fine on every other WiFi connection I've used, including my home wifi.
Sometimes it is indeed bad luck of the draw. The WiFi built into the 6p apparently just doesn't play well with your router. At least that is my guess. Try other places and see what happens before you return.