@ZolaBA 

I know I am not answering your question, but in your setup it would make sense to run Deco mesh in Access Point mode, with Asus rt-n18u managing the network. 

I can't tell why Asus rt-n18u router won't be able to handle gigabit wired network traffic. Are you unhappy with that router for any reason?

Also, you do run Deco mesh in Access Point mode, right?

@Alexandre. Thank you for answering!

So, yes. Now Decos just run as access points, and the asus makes the routing jobs (nat, dhcp, etc. - except AP funkcion). 

Unhappy? absolutly not, everything is fine! Maybe i just want better - if it possible :) 

@ZolaBA 

I have similar network setup: I kept my ISP router, with its WiFi off, and run Deco in Access Point mode. I do like Deco as a WiFi mesh, but if you read this forum you'll see that not everyone likes Deco as a router.

If you are still willing to replace your Asus router, run test with Deco units you already have. Change your Deco mesh to Router mode with M4 as Main Deco. Chances are, M4 will handle Ethernet gigabit traffic just fine. 

From models that support WiFi5 and similar in functionality to M4 you have, the most powerful is M9Plus. I have one of those and it is my Main Deco. I like it, but its compact design could make heat dissipation a problem. My M9Plus is quite warm even on light usage, it may get hot or too hot on the network with heavy load, which could impact Main Deco performance.

If you want something more capable, check X20 or X60. Yet, these units support WiFi6 and rest of your home network does not. That might create a situation with WiFi6 capable mobile device(s) not roaming properly from X20 to the rest of your Deco mesh. Not saying it will happen, but it might.

By the way, you may have to redo your home network if you switch Deco mesh to Router mode, and specifically in your case where every Satellite Deco unit is wired. Review this document to see what limitations you'll be facing with wiring Satellite Deco for mesh in Router mode: General questions about Ethernet Backhaul feature on your Deco.