MichaelL wrote

I'm having trouble figuring out why i'm not able to get my unit to power on with the ethernet cable to my router, as a PoE solution.

You need either a PoE-capable switch (such as TL-SG108PE for example) or a PoE injector (such as TL-POE150S), which provide 802.3af PoE.

It's a PoE accesspoint, but it will only work with the power adapter.

Hm. My EAP225 works fine with PoE. Just to make sure: Ethernet is not USB, standard Ethernet does not have PoE capabilities. PoE-capable switches use special techniques to support power over the Ethernet wires, that's either PoE according to IEEE 802.3af (15.4 watts per port) or IEEE 802.3at (up to 30 watts per port). See http://www.tp-link.com/common/Promo/en/POE/index.html for an explanation.

Also, is it possible to make it act as a extender so i can use the gigabitethernet port for a client PC that does not have any Wifi networkcard and then just power it with the power adapter?

No. The EAPs are just APs and currently have no client mode. I would recommend an USB WiFi adapter for a client PC w/o WiFi network card.