@Maurits_1971 You are welcome. I also have Deco M9 Plus in my Deco mesh, so I am quite familiar with them. Two final recommendations: 1. If you haven't done it yet, visit Update Deco page in Deco app...

Start with the oldest. If successful, you will not need to use this method for next firmware upgrade: you can do it through Deco app. You are using this method only once. After that, it will be...

@WindowsNT_Cork Do not enable Guest Network - and if you already did, then disable it back. Having Guest Network enabled will not do any good: these mysterious devices are connected to your Main...

@Maurits_1971 As I thought: firmware 1.2. Also, your M9 Plus is hardware version 2. Follow this document to upgrade your M9 Plus: How to Update the Firmware of Deco Try Method 3 and if that does not...

@Maurits_1971 I saw this happening when Deco M9 Plus has very old firmware, version 1.2.*. I can't tell if all devices connected to Main Deco is just a reporting issue or they indeed only connect to...

@cjferoussis I am not from TP-Link, so I would not know, but even TP-Link Support that checks this forum is not privy to that info. I can see you have M4(R) hardware V2 and S4 hardware V2. You may be...