@DanieleDB 

Hi,

You cannot have a Mesh network with Ethernet Backhaul between the Archer AX55 Pro and the RE700X. None of the two devices supports this feature as of today.
But you can keep an eye on the first posts of the following two threads for any updates on Ethernet Backhaul for your devices.

https://community.tp-link.com/en/home/forum/topic/593722

https://community.tp-link.com/en/home/forum/topic/644776

What you could do is to connect the RE700X to the AX55 Pro via Ethernet cable, configure the RE700X as "Access Point" and set the SSIDs of its Wi-Fi bands to be identical to those of the AX55 Pro's.
A disadvantage of this setup is that roaming between the AX55 Pro and the RE700X might not work well, as in this case it depends solely on the client device itself.
 

  @woozle are you sure about it? I am 100% sure the RE700X can do it.

i checked reviews of this products and instructions. What I am not sure about is my router.

  @DanieleDB 

If the RE700X indeed supports EasyMesh via Ethernet Backhaul, that would be great news. Quite a few people here have been asking for a range extender with Gigabit Ethernet port to support EasyMesh via Ethernet Backhaul. Can you maybe share the reviews and instructions that you've found? 

But in general, only the devices listed in the two threads I posted earlier support EasyMesh via Ethernet Backhaul. And I don't see the Archer AX55 Pro listed there.