@MrRicciolo 

I would recommend configuring Deco mesh to run in Access Point mode. That will put everything on 192.168.1.xxx network and allow all devices on your home network access printer.

See Switching a Deco to Access Point (AP) Mode

  @Alexandre.

Thank you. In fact when I was in AP mode I didn't have any problems. But now I need a configuration working in router mode.

I could also connect the printer to the 192.168.1.xxx network via cable, and everyone could print (including wireless users). I did this test and it works.

However, it would be better for me not to have to move the printer and so I was wondering if there wasn't a solution by playing around with the Deco settings.

  @MrRicciolo 

1. Have you tried connecting printer to ISP router by WiFi on 2.4GHz band, or are they too far from each other even for 2.4GHz?

2. Port forwarding is common way to access printer behind the router, which is what your Main Deco is. I think you would need 9100 port. See How to set up Port Forwarding feature on the Deco Any issues with that? 

Make sure you use IP reservation feature in Deco and not configure static IP address on printer itself.