@Hansvdc 

What is the firmware version of your M9 Plus?

You can find this information in Update Deco page of Deco app.

Example:

1.2.11 Build 20190926 Rel. 55727 . The app says this is the latest version.

Hansvdc wrote

1.2.11 Build 20190926 Rel. 55727

  @Hansvdc 

That's may be where the problem is. Firmware 1.2 is from 2019, almost 5 years old. As you can see from my example, current M9 Plus firmware is 1.6.

Fiirmware 1.2 is so old Deco app can't update it through Update Deco page. App mistakenly says "Your Deco system is up-to-date," instead of "Unable to update."

You must do manual update. Use Method 3 from the following TP-Link document: How to Update the Firmware of Deco

You only need to do manual update once. If successful, Deco app will be able to update M9 Plus firmware in the future. 

ok just did the update, apparently I'm now on firmware 1.6.3 build 20231225 rel 60770. Would that be ok?

  @Hansvdc 

Yes, that should be OK. Check Update Deco page - if all your M9 Plus are on this firmware and none left with firmware 1.2, in the future you can just use Update Deco to check for new firmware and to update your Deco mesh.

With firmware 1.6 your Deco mesh got multiple new useful features, better stability and also the following alert: Abnormal WAN Port Speed. Please Check Your WAN Port Connection

Next time you see M9 Plus losing speed, check Deco app for that alert. If it is there, you'll need to replace Ethernet cable.

Thanks a lot! Always assumed I was on the latest firmware because the app told me so... Learned something new. Would love to use my device as a router instead of an AP, but I think I cannot do this because some devices are connected straight to the ISP's router and would not be able to "see" other network devices which are on the Deco's AP. For example my TV, PS5 and cable box is connected directly to the ISP router, while my pc is connected via the Deco 'AP'.