Did you check IP address obtained on the device connected to WA901ND? DHCP has been disabled on WA901ND, right? MAC filter is enabled on the main router? If you disable it, does the bridge work?

Bypassing powerline devices, can you still access internet? Have you tried to restart all devices? If direct connection to the hub works, try to reset powerline devices and re-pair them.

What's the firmware version of your WPA4220? The airprint issue has been fixed with firmware 131219. http://www.tp-link.com/en/support/download/?model=TL-WPA4220&version=V1#tbl_j

When the problem occurs, how about status of lights on powerline devices? Fix different channels for each of Wi-Fi connections.

Plug the two units in a power extension cord or place them in the same room to try. Actually, there is another way to set up a private network. You can rename the network using utility....

PA210KIT and PA4010KIT are compatible with each other. You have no computer? If so, you can try to pair them. When you pair, you have to press pair button on PA210 for about 5 seconds but press pair...