@GoncaloMendes 

May I know how many Deco M4 in the network, and how are they connected to the network? It would be helpful if you could provide more details, then we can try to work it out, thanks.
1. Ensure the cables are working fine, which should be cat 5e or cat 6 cables. You may try with different Ethernet cables.
2. Please also ensure you can reach 1000Mbps when connecting a computer directly to the ISP modem (the same LAN port where the main Deco is plugged in), this is very important.
3. Please check the QoS settings on the Deco app, ensure it is correctly configured. You can also refer to these suggestions:
https://community.tp-link.com/en/home/forum/topic/254396
4. Is there a way to check the negotiation speed on the ISP modem web page that if the main Deco M4 negotiates at 100Mbps or 1000Mbps? Try to swap the RE Deco nodes with the main one to check if this will make any difference.
How to Setup the Satellite Deco to be the Main Deco? Check this guide:
https://www.tp-link.com/support/faq/2400/