@AndreCasarin If you had Deco E4 with 100Mbps Ethernet port, that would be expected outcome. Deco M4R has gigabit Ethernet ports and what you describe would be unusual for it. Deco E4 is very...

@moo_tada From screenshots you provided I can see that the status of your 3rd Deco is not OFFLINE. You have removed 3rd Deco from your Deco mesh. This was supposed to reset 3rd Deco and make it...

@moo_tada You have two Satellite Deco side by side, one shows as connected to Main Deco and another shows as offline. Please do the following for me: Bring both Satellite Deco to where they both can...

@moo_tada It is possible that 3rd Deco is too far from Main Deco to connect to, or your 3rd Deco experienced hardware issue. If you bring your 2nd Deco to that same place on a different floor, where...

@Smejkal Second option will not work with Deco in Router mode. Check this TP-Link FAQ: General questions about Ethernet Backhaul feature on your Deco

@Golfetta Different M4 hardware versions is the problem. TP-Link only has M4 V4 firmware that allows to configure how Deco connect to each other. When you added M4 V2 to your mesh, not only they do...