@Ppmh 

Hi,

Apart from the router supporting EasyMesh in AP mode or not.

If I am not mistaken (someone please correct me if I am wrong) the Ethernet connection colored in red in the sketch below cannot be used as "supply line" and Ethernet backhaul simultaneously. To make wired EasyMesh work the Ethernet cable colored in green that is coming from the Satellite router would have to be rerouted to connect to the Main BE550 router instead of the ISP router.

  @woozle 

Thanks.

I had been looking at topo 3 of https://community.tp-link.com/en/home/forum/topic/697572 but I guess you are right and it will not work in my case. 

Would the alternative be the setup below? (W is WAN port, L is LAN port) Or should the sattelite routers be connected via the LAN port?

However, such a setup would require me to buy another Router, unless there is an alternative method. 

I am not an network expert but I do wonder what it means when an router is configured as a sattelite unit. Will it still operate as a full functioning router (including DHCP, NAT)? Normaly such a sattelite device would be set in AP mode, but when I do this it does not support EasyMesh. 

I wonder whether this will work at all and if I shoud have bought a Deco like solution. 

  @Ppmh 

I see. You are right. According to the post you linked to your original idea (Topo 3) should actually work. Thanks for pointing it out. I didn't know that.

The satellite units will always work similar to AP mode, they will not add a NAT and also don't perform as DHCP server. 
When you prepare an TP-Link EasyMesh router for being added as a satellite unit you don't switch that router to AP mode. You need to switch it to "Satellite mode" and then leave it like that (i.e. don't log into that router again before it's been added to the EasyMesh network).

A router that is to be used as satellite unit doesn't need to support that "EasyMesh in AP Mode" feature that you have read about. This feature is only important for the router that is used as the EasyMesh main unit.
 

  @woozle 

Thanks for explaining that.

I guess in stead of using a BE550 as main router, I could also use a BE230, that supports AP mode in Easymesh. The Wifi fornthis device is turned of anyway.  That is cheaper solution and I can leave my current ISP router in routing mode.