TD-W8951NB is brand new or it was working with your line? It is a synchonization issue between the modem and your ISP line. If the line works with another router, it should be a problem with W8951NB....

To prolong the lifespan of the battery, we'd better charge it after the battery runs down.

What's the hardware version and firmware version of your WR941ND? How did you connect the router and two range extenders? The second WA850RE is extending Wi-Fi signal from the first WA850RE or the...

When it shows limited connectivity, what's the IP address obtained? Update driver of wireless card on your laptop. Try to configure static IP.

Yes, any one.

Upgrade latest firmware first. http://www.tp-link.com/en/download/TL-WR941ND_V5.html#Firmware After upgrade, reset it and reconfigure it to see if the problem continues.