Directly connecting to Linksys works OK all the time? Try to use C7 as an access point to see what happens. http://www.tp-link.com/en/article/?faqid=417

D7 has no one to one NAT feature. Before you buy it, you didn't confirm it?

How about status of Wireless 5GHz light? On status page, 5GHz is disabled but 2.4GHz is enabled? Can you detect 2.4GHz network? Under Wireless 5GHz, do you mean that "enable SSID broadcast" is...

Here is the instruction. http://www.tp-link.com/en/article/?faqid=399

Do you mean access control or something else? But it seems that it doesn't have inbound services which can be blocked or allowed by telling the service name.

Your printer is connecting to powerline adapter or root router? Is it a wireless printer? You'd better provide topology.