SG108 is an unmanaged switch, plug and play. Is it SG108E?

Is it the same problem here? http://www.tp-link.com/en/faq-551.html Give a try. If it doesn't help, provide model number and firmware version of it, as well as model number of your main router.

How about status of all lights on front panel of WR941ND? What's the IP address obtained on your PC connected to LAN port of WR941ND?

It is not quite secure to remember login username and password... What kinds of web browser do you use?

80 is the default web management port of WR841N. If you cannot change port of NAS, you can change the port on router page to avoid conflict. Go to Security---Remote Management. After change and save...

Report the problem here. http://ticket.tp-link.com/index.php?/Tickets/Submit