@Thomas-Deco-M9P To update M9 Plus firmware 1.2 see this document: How to Update the Firmware of Deco Follow Method 3 from it.

@Angstzaaier Just to note, I can access M9 Plus Web interface using Microsoft Edge on Windows. You have quite unusual issue. I don't work for TP-Link, but if I did, that would be my line of thought...

@Thomas-Deco-M9P Deco M9 Plus firmware 1.2 is four years old. It is so old it does not have ability to be upgraded through Deco app. Instead of telling you it can't update M9 Plus firmware, Deco app...

@mamypoko Deco M9 Plus firmware 1.2 is four years old. It is so old it does not have ability to be upgraded through Deco app. Instead of telling you it can't update M9 Plus firmware, Deco app says...

@OneFromFL TP-Link states not to do that if Deco mesh runs in Router mode. Which is also what I told you: this configuration only works when Deco mesh runs in Access Point mode. I have three Deco,...

@Angstzaaier I, too, have M9 Plus in my Deco mesh. It is my Main Deco. I shared screenshot from M9 Plus with you. I run Deco mesh in AP mode. I may have an idea why you can't access M9 Plus Web...