TL-WA801ND don't have management VLAN, the packet from device itself is untagged. And if the TL-WA801ND receives packet from different VLAN, it would transfer to the right VLAN. But not itself, so...

It doesn't support management VLAN. And i know that TP-Link is working for that now.

I think TP-LINK WA500G is too old that you cannot find it in official website. Maybe you could change the wireless channel, use different SSID and fix the wireless bandwidth to 20Mhz for the test...

You may could use inSSider software to find the idle channel that have less wireless signal. Then you could fix it to the idle wireless channel and use 20MHz as its wireless bandwidth. Just avoid the...

I am sorry that there are not 'Max Tx Rate' in the latest firmware. I think it would be the max.

If you want to let 901ND gets IP address from VLAN20, you just need to make sure that untagged packets from 901ND could go to VLAN20. That would be fine. But there are no way to only allow VLAN20...