@RJT463 You can add M4. They will be communicating with P9 by WiFi, not by powerline obviously as M4 don't support powerline connectivity, but other than that they'll work just fine together with P9...

@Vectorlost Things to check: 1. I wonder if any of your wired devices have static IP address configured on them, and occasionally PC connected to Deco 3 gets into IP address conflict (or Deco 3...

@RJT463 Yes, additional Deco M4 will work together with existent M4 you have, no problem. With Deco, there are no "primary" and "satellite" Deco units different in hardware and design. They all are...

@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...

@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...

@Canidothis You can buy simple 5- or 8-port gigabit networking switch, connect it by Ethernet cable to one of M4 Ethernet ports, and connect your wired devices to switch.