Which Firmware you are using now? How about downgrade to the old and original Firmware?

Compare with installing exchange an antenna for you wifi adapter, I suppose to do something with your AP or wifi router. For example, installing an high gain antenna for you AP/wifi router, adding...

TL-WN422G v1 is a really old product, based on Atheros AR2524 (ZyDAS ZD1211B). Even the chipset vendor Atheros has stop to research drivers for it, not mention to manufacturer TP-LINK....

http://www.tp-link.com/resources/simulator/TL-SG2424/index.htm http://www.support.dlink.com/emulators/dap2590/index.html Try to enable VLAN on D-Link DAP-2590, and set up to be one VLAN with TP-LINK...

TL-SL3428 and TL-SL2428WEB Power Supply Module: 100～240VAC, 50/60Hz Input; 12VDC/2A Output.

In general, I suppose the max No. of wireless clients connect to a SOHO class AP is around 10~15....