@l0bit0 

You will need to relax your requirements:

1. Run Deco mesh in Access Point mode;

or

2. Run Deco mesh in Router mode and treat Main Deco as your router (as you should), connect Home Assistant Server to Main Deco LAN port (buy switch if Main Deco LAN port is already taken). You can also connect Home Assistant Server to any Ethernet port of any Satellite Deco. If you can't move Main Deco, you may be able to move Satellite Deco close to Home Assistant Server.

  @Alexandre. Thanks, I suppose I did not had too much options....

anyway I assume should've be more easy as at the end both routers ar physically connected...

anything can be done wit port forwarding or any other router option?

thanks

  @l0bit0 

Port forwarding is pain in the neck on a good day, but in your case it is unlikely to work. Port forwarding maps port to a single internal address. If your HomeServer needs to talk on the same port to multiple devices connected to Deco mesh, you can't do port forwarding. 

In theory, if every device on Deco mesh to which HomeServer needs to talk listens on different port and each configured with reserved IP in Deco, you could create port forwarding rules for all of them. 

This is all not easy because for Main Deco everything which is outside is Internet. It considers your router to which Main Deco is connected an Internet. You would not want a random device from the Internet have unrestricted access to your home network. Nobody would, so bypassing basic firewall protection often is pain in the neck. It is by design.

Bring Satellite Deco to where HomeServer is and wire HomeServer to Satellite Deco. Do that just to test it works. If it does, either keep Satellite Deco there or buy single X20 and add to your Deco mesh to support HomeServer link. 

Also consider other options where HomeServer can be connected to Deco mesh. Having everything under same router is always better and could save you from much of aggravation in the future.