@serppower A little more information would be helpful.  The EAP is wired directly to the Archer?  The desktops are all wireless? Not trying to be smart, just knowing exactly how its configured is the first step in figuring out what could be wrong.

  @RickJamesBish 

The EAP is wired to the POE adapter POE port and the Router is wired to the POE adapter Lan port

but it keep connecting and disconneting from the router I don't know why.

For the config I just accessed it through the web interface and then set user and pass and set a new wifi 2.4 and 5 ghz networks SSIDs, that's it

and I noticed that the memory usage is 52% all the time.

I really suspect the cable but I am not sure, Do you recommend the current Cat5e or better get a Cat6 cable? which is better for POE?

  @serppower I would check my ethernet connections and make sure they are plugged all the way in, not just visually, pull without depressing the release tab and make sure none pull out. If possible watch the LED indicator on the adapter if it has one and see if it drops out, meaning the AP is losing power then coming back on. Make sure the power cord where it plugs into the POE adapter is seated well and that the receptable end is keeping constant connection. If those are all check out ok, then I would suspect the ethernet cable itself. 

I did have a similar issue and I had a ethernet end that had a broken release tab so the connection was intermident.

  @RickJamesBish 

Thank you Rick, I will follow what you have said and will let you know the progress.

  @RickJamesBish 

I applied your steps and found out that the main cable between POE adapter and EAP is not good so I will replace it.

Can I run CAT6 cable instead of the CAT5e? Will it cause problems? The distance between the POE adapter and EAP is just 15m.

  @serppower 

Yes you can use Cat 6 cable, it's better to use STP shield Ethernet cable.

You may also check the cable between your router and the poe injector. It cannot be a long cable.

  @Somnus 

Thanks for your help, The cable between router and Poe is 6 meters cat6, Is it good or I should try to make it shorter?

  @serppower 

It's now working, It was the cable between POE and the EAP access point was bad and replaced it with a CAT6.

Thanks for your help guys.