all machines connected to the old network uses the DHCP from the main fiber-optics router What's the IP address and default gateway obtained? When C8 is working as an access point, you should disable...

You want to block all internet access? Not only download? If so, refer to the thread here. http://forum.tp-link.com/showthread.php?82264-Blocking-Access-to-Internet

Can you update driver of the wireless card on your phone? Any other wireless clients like a laptop or tablet which gets a normal speed? The router is a brand new one? Did you reset and reconfigure...

I don't think WR941ND can handle so many clients at the same time... It is a SOHO router...the maximum number of wireless clients is about 15...

I need a good strong wireless signal as we live in an old property and the walls seem to block the signal pretty well. If so, maybe you need powerline extenders or range extenders to extend Wi-Fi...

Is it GPT or MBR partition? How about formats of the hard drive? There is an instruction but not sure if there is any update... http://www.tp-link.com/en/faq-603.html