@Soundman32 

If you connect WAN/Internet port of Archer C20 to a modem via an Ethernet cable, and then want to connect EAP to C20's Wi-Fi, that is impossible because EAP only has access point mode.

But if you connect EAP to a router via an Ethernet cable, and want to connect C20 to EAP's Wi-Fi, I think it's worth a try. Not sure if they are compatible.

@Somnus "But if you connect EAP to a router via an Ethernet cable, and want to connect C20 to EAP's Wi-Fi, I think it's worth a try. Not sure if they are compatible." - that's what my question is asking.   If it is possible, how do I do it?

@Soundman32 

Just to speak honestly, while im not familiar with the C20i looking at the spec of this device, honestly would recommend you don't go down this path and look to replace the hardware

WDS doesnt require support a both sides, if you do set this up WDS is only required as the client side (the C20i).  The 225 will simply treat this as any other device connected to the SSID, blissfully unconcerned its providing x number of others, it will simply see it as "one client".   The WDS is handled by the c20i and should work

However...  WDS should be done at 5ghz for maximum transfer, the c20i only supports single 1x1 433mbps connection and would likely suffer massive performance due to that.    As there is only 1 5ghz radio its going to have to balance the backbone and clients on that single antenna, that's 50% loss straight away and likely more once you add overhead/interference.   I would suspect you are going to get 150mbps max in WDS..   honestly not worth it!

In WDS you ideally need a 3x3 (or 2x2 min) device, a 3x3 can dedicate 2x2 to the 5ghz backbone (866mbps) and have a spare radio for (433mbps) to serve clients (provided you have mimo support).  

However WDS is pretty dead now as a technology, you would be much better buying a cheap EAP225 / 245 and setting up a MESH network.. but you would require a controller also (software or hardware)..   Cheaper option would simply be to buy a plug in repeater, something like the RE range extenders