@KoenCrabbe 

Given how the printer is only able to connect to 2.4GHz, it might be getting confused since both the 2.4GHz and 5GHz are the same. If you disable the 5GHz, reboot your printer are you then able to print?

I would also recommend reaching out to the printer manufacturer to see if they have known issues with mesh networks.

@Tony Hi Tony, thanks for your reaction,  I will do this.

In the meanwhile I  descovered some other posts and I further did following things:
- factory reset of the printer: it gets now an IP adres in the good range where the other wifi devices are: i.e; 192.168.68.116 => I can ping it now and in the browser I see the homepage of the printer

- connected it to the guest network, only allowing  2.4 Ghz. Again it gets the good IP adres, but I can't discover it via the Canon software

- in both cases, it doesn't appear in the device list in the Deco app

@KoenCrabbe 

Since the printer is now connected I would enable the 5GHz now. Test the printer afterwards if it still works, and if it does then you will be fine leaving the 2.4GHz and 5GHz on.

@Tony 

Hi Tony, in the meanwhile I tested it and it works only for a short while on 5Ghz. In parallel I checked on the Canon forum and it is only 2.4 Ghz capable.

So if I leave the mesh on 2.4Ghz what is the impact on throughput and performance?

@KoenCrabbe 

The impact you will see depends on your internet speed. If you have internet speeds say below 200 Mbps, then you will not notice anything, but if you have 1 Gbps internet then you may see less then half that speed.

What you can also try to do is to enable the guest network, turn on only the 2.4GHz, make the wireless name different and disable the 5GHz. You will also need to enable the option for the guest network to access the main network so your devices can print to it. The only thing is you will need to set up the printer to connect to the guest as doing WDS will connect it back to the main network. In the end you will be able to have the 2.4GHz and 5Ghz enabled on the main network and the guest network for your printer.

@Tony 

And how do I enable the acces from the guest network to the main network?

@KoenCrabbe 

My apologies, if you are looking at the guest network settings and do not see any other options other then the name and wireless password it is probably not an available option. 

The guest network would be found by selecting the Wi-Fi option in the Deco app:

You can continue to leave only the 2.4GHz enabled to see how your network performs based on your needs.

@Tony 

Indeed this option does not excist for M9 Plus; Could be solved with a software change in the Deco app I presume...

I leave it as it is for now. We can print.

My next printer will have a Lan port for sure... ;-)

In the deco app my download speed is 60Mbps, about the same as on my wired PC.

Thx for your help

@KoenCrabbe 

You are very welcome, sorry about the mix-up.