@Kenta49 

M9 Plus firmware 1.2 is from 2019, soon to be 5 years old. It does not have Web interface. It does not accept firmware update through Deco app.

You must update firmware manually to newer version to get WebUI access, gain ability to update Deco mesh through Deco app, and have very many stability and feature improvements.

Latest Deco M9 Plus firmware is 1.6.

To update M9 Plus firmware manually, follow Method 3 from this document, that's the only way: How to Update the Firmware of Deco

  @Kenta49 

Hi. This is what I get when I try to do it your way.
How to go forward from here?

I´m on the same wifi and no VPN and not even VM.

If there is any problem with owner andmanager.
How can I become the owner of the network?

Kind regards Kenta49

  @Kenta49 

According to TP-Link Deco FAQ-2930 this error message happens before user ID and password are verified. It has nothing to do with owner/manager of the network. The app scans network and can't find Main Deco.

There are three troubleshooting steps in FAQ-2930 for that error message. Have you tried them all?

If you did, find another device to run update app on it. Find desktop/laptop with Ethernet port and connect it with Ethernet cable to second Ethernet port of Main Deco.

Other people tried this update app and while not everyone was successful, overall success rate is high. As I said, this is the only way to update M9 Plus with firmware 1.2.