@LasseN 

Below is the instruction to enable WPS feature on Deco.

https://www.tp-link.com/support/faq/2332/

If it does not work, try to reboot the Deco and the printer. 

Still the same, try to connect your network printer to the Deco with wifi password. 

May it help. 

@Kevin_Z Thanks for reply!

Have tried the two first options (should have mentioned that of course, sorry)

Connecting with WiFi password should not be necessary with gear featuring WPS.

Is WPS generally an issue with DECO or could there be other settings I miss?

For example, is there in this matter and difference if I set them up i router or AP mode?

@LasseN 

You are right, we do not need connect with wifi password for those devices which supports WPS. While cause the WPS mechanism varies due to the different products/manufacturers, it may fails. 

While most devices can be connected to the wireless network via wifi password, that is why I suggest you to connect the network printer to the Deco by wireless password.

If both failed, try to disable fast roaming and setup QoS as standard and clear all the high priority rules to give it a go.

Besides, you can disable 5GHz on the Deco app. 

Good day.