@taxizhs82 

There are SO many moving parts in your setup that you did not discuss it would be impossible to provide you a fix right away.

My suggestion is to work with a single AP at building A and a single AP at building B (do NOT power up the second APs), and set them up ideally so they have 'Line of Sight'...ie you can see the other AP from the mounting position of the local AP.  Next, based on your country, you need to choose and then fix the available 5.8Ghz channel on the ISP router side EAP on a channel that permits you to send the highest amount of power across it (if you can figure this out, please share your country on the forum).  Test speeds with a laptop which directly connected to the remote switch which is feeding the EAP (the laptop will appear to be plugged into the same LAN port as the ISP side AP).  That is your theoretical maximum based on local conditions.  From there we can discuss how to optimize the use of the second AP at each site.

A birds-eye view diagram of the buildings, AP locations, approx distances and whether there are obstructions (trees, other buildings, hills etc) would be useful.

  @taxizhs82 

EAP225-outdoors only mesh on 5.8Ghz, which doesn't propagate as well as 2.4Ghz.  What frequency was your Ubiquiti setup using?

What channel are the APs currently meshing (poorly) on, and what country are you in?

  @taxizhs82 

Your best bet in EU/Greece is the UNII-2c (channel 114 is center channel in this band).  It is required to defer to radar signals so that may be another wrinkle.

There is no option to mesh (wireless backhaul) on 2.4G with TPLlink APs, you need a different vendor/technology solution to do that.  Honestly, I would stick to a Ubiquiti solution for the building-building link, and then run the Omada WiFi solution over top of that.  That would give you the best of both worlds.  Something like the UBIQUITI LBE-5AC-GEN2, which is 5Ghz based, but directional and should get you 250-350Mbps at the far end.  With some extra 'drachmas' maybe Ubiquiti UBB, UniFi Building-to-Building Bridge (2-Pack) which I know is available in Greece because it's 60Ghz based.