@Bjebeje 

Just in case, power off/on all M9 Plus and check again in Deco app. 

If Main Deco M9 Plus continues to stay on firmware 1.2, do the following:

Promote one of updated Satellite Deco M9 Plus to Main Deco: How to replace the main Deco with another Deco

Delete from your Deco mesh Satellite M9 Plus which used to be Main Deco and stays on firmware 1.2. For that, follow Reset a Deco unit, Method 1 from this document: How can I reset or reboot my Deco Whole-Home Wi-Fi System?

See if you have now correct reporting in Deco app.

YAY  @Alexandre.!

Connected to the uncooperative Primary unit with a LAN cable and did it all over again.

NOW it updated!

AND devices do connect to the Deco nearby.

Many thanks for your support and patience.

Make a wonderful day 

  @Bjebeje 

Great! Will keep that solution in mind when someone else has issues with M9 Plus firmware 1.2 update.

Now your Deco M9 Plus must be on firmware 1.6. You can check that in Update Deco page. With both M9 Plus and M5 fully updated, you have unlocked all new features introduced in firmware 1.5 and 1.6, see the list here: Understand the Differences Among Deco Firmware Platforms

Enjoy.