@ExpressoCup What router? Is that a POE Router or are you using the supplied POE adapter that came with the EAP225 OD?  Some of the POE adapters, even the ones that come with TPlink products, are limited to 100 FD, even though the device is capable of more.  The one that comes with the EAP225 OD is suppose to be 1000 FD, but check the model number of the adapter and look up specs to make sure you received the correct one. 

 I know you would prefer that it detect this automatically, but have you checked in the negotiation setting on the router and see if you can change it from AUTO to 1000 Mbps FD?  Test and see if you are getting the desired throughput and if so just leave it manually set.

I am using the POE adapter that came with it, I didn't find any markings on it other than TP-Link, so I don't know how I can identify it)

I use a router from my ISP. In his console TP-Link device uses half-duplex and I can not change it in any way. If I connect the LAN cable from TP-Link directly to the PC, the connection status also shows 100Mbit speed.
It seems that the problem is in the POE adapter, because according to the specification TP-Link has a gigabit port after all

  @ExpressoCup  yes it could be POE but that EAP is suppose to have a 1000 port on the POE.  What HW version do you have?

  @ExpressoCup have you run a test to see if it is actually limiting to 100? I am not sure what else you can do to verify the POE adapter is not the issue short of trying another one. Sorry I could not be more help.

The port is really limited to 100mbit, speedtest confirms 

I tried to connect TPlink to the computer via LAN and the connection status is also "100mbit". So, ordered a new POE adapter) When it arrives, I'll let you know the results. Thanks for the help!

No, the situation with the new adapter is the same :(

  @ExpressoCup if you have eliminated the POE adapter and the cable then it falls to either the EAP or the Router and a negotiation issue. Since I dont know what router you have you will need to look up the possibility of manually setting the port speed.

  @RickJamesBish router is NOKIA G-2426G-A WiFi 6 with a fiber optic connection. In its console, the operating mode of the port to which the EAP is connected is "Full-Duplex" with a bitrate of 100Mbit. If you connect PC to the same port with the same cable, the bitrate will be 1000Mbit. There is no manual setting :(

  @ExpressoCup Then you are restricted to either TPlink or Nokia addressing the issue with firmware.