@Macca14 

Deco model that you have, is it Deco E4?

  @Alexandre. Thanks Alexandre, yes it's a Deco E4.  Do you have any idea of the problem? 

  @Macca14 

If you check hardware specs of Deco E4, you will see it has 100Mbps (Fast Ethernet) Ethernet ports, not gigabit. Main Deco E4 wired link is what limits Internet speeds.

  @Alexandre. 
 

Thanks again Alexandre. It says in the box that it has 1200Mbps dual band wi fi. 

  @Macca14 

Check this link and scroll down to "Ethernet Ports" section: https://www.tp-link.com/en/home-networking/deco/deco-e4/#specifications

It does not matter how fast wireless speed of Deco E4 is when you run speed tests using Internet servers, what matters is speed of wired link from Main Deco to modem/router. The wired link is 100Mbps max.

If you manage to connect two devices wirelessly over WiFi 5 to same Deco E4, and transfer data directly between these devices, the wireless transfer speed will be way faster than 100Mbps.

  @Macca14 

Suppose you have PC connected wirelessly to Deco E4, and file server also connected wirelessly to same E4.

If you try to copy file from file server to PC, data transfer speed will be faster than 100Mbps. It will be limited by wireless capabilities of Deco E4.

When you accessing Internet through Deco E4, all traffic goes through wired link from Main Deco E4 to ISP modem/router. The speed of that wired link will not exceed 100Mbps, because it is the speed of Deco E4 Ethernet port. 

  @Macca14 

This is correct. Other options are M5, M9 Plus, S4 and S7.

Any of them as Main Deco will unlock faster speeds.