@noctropolis 

Hi,

Is there a particular reason why it has to be a "Router" with Range Extender mode?

TP-Link is now also offering a couple of range extender models that can be placed on a table instead of being one piece with the wall plug.
https://www.tp-link.com/en/home-networking/range-extender/

  @noctropolis 

Yes, I want a better router with Wi-Fi extender mode to replace my TL-WR841N. Better antennas, better configuration options, the possibility to easily connect some devices by cable on my floor. Would be C54 good choice for me, or it is any better?

  @noctropolis 

 C54  MediaTek MT7628DBN 580 MHz Flash 4 MB  RAM: 16 MB

C64 Mediatek MT7626 1.2Ghz Flash 4MB RAM: 32MB

I use C64 as repeater. works perfect. 

enable wds bridging

disable dhcp

  @noctropolis 

wifi boosters are hopelessly outdated and do not provide the same smooth transition as modern mesh models

so i would recommend you to give up amplifiers and use mesh technology

The second nuance is in your layout.

connecting mesh points via wifi works fine when there is direct visibility of the points and there are not many of them (2-3 pieces)

in your case you will have to run a cable between floors if you want to get a stable connection

here a person has exactly the same problem:

https://community.tp-link.com/en/home/forum/topic/714030?replyId=1435622

so sooner or later, you will still come to the conclusion that you need to lay a cable between the points

this is the only right and correct decision if you want good results

  @YuriyB 

Thanks, but according to specifications on the official TP-LINK site, the C64 model doesn't have a range extender option, the C54 is the highest model with that option.

  @noctropolis  you can try, WDS.

It work like range extender