On DHCP---DHCP Settings page, Lease Time is 120 minutes? Try to change it. Can you use static IP as HiSpeed suggested?

Directly connecting to your modem works or not? Wired connection to C8 works or not? As to the wireless issue, can you describe in details? All wireless clients can detect the Wi-Fi network, right?...

Did you connect the modem directly to your PC and keep testing it for more than 45 minutes to see what happens?

Can you still login web interface of RE200? Upgrade the latest firmware and reconfigure it. http://www.tp-link.com/en/download/RE200_V1.html#Firmware

Try the driver for Windows 8 or 8.1. Driver for Windows 10 has not been released yet but it will... We have to be more patient... http://www.tp-link.com/en/download/TL-WN821N_V4.html#Driver

TL-WN851ND is old and it seems that it even doesn't support Windows 8 or 8.1. Hard to say if there is any update for Windows 10 in future...