Can you share a screenshot of the error mark? If the LED is green, internet should be working. Can you still connect your computer to the deco unit and access internet? If it's just the app that's...

You can check the link speed on the computer. Even your router supports 11ac, if the channel width is low, you won't get the maximum 11ac speed.

Is your printer on the compatibility list? You may try reinstall the USB utility on the windows 10. And most importantly, make sure the printer works when connect to the win10 computer directly.

Do you see traffic statistics under system tools? That will show you the total and current bandwidth traffic of devices connected to the router, but on daily scale. Maybe you can set bandwidth...

You mean the target for access control? Usually there is a limit to the amount of target, or host, or rules, some are 16, some are 32. There is no workaround unless they release a firmware that will...

What's the speed if you connect laptop to the router through Ethernet? If only WiFi is slow, you can change some settings on the router. http://www.tp-link.com/faq-1077.html