Deco M9 plus with Harman Kardon Citation 300: issue with connection loss
I have a Deco M9 Plus downstairs which is about 4 metres away from my smart speaker Harman Kardon Citation 300. The Citation 300 gives a 'weak' wifi signal in it's display. The device losses connection sometimes. Then I have to reboot it. Then I turned of the 5g band in the Deco M9, and with 2.4g the Citation 300 receives 'excellent' wifi signal. No signal loss and so no connection loss. But I don't want to set the Deco M9 to 2.4g only, for obvious reason's. I have another Harman Kardon Citation One device approx 6 metres form the Deco with no issues at all. The Citation 300 is allready replaced with a new one, but same issue.
My idea was to set the Citation 300 to use 2.4g only, but after discussing this the support of Harman Kardon, this isn't an option.
Can I somehow set the Deco M9 so that it only supports 2.4g for this specific device and the rest will work with 2.4g and 5g?
Note that my Deco app doesn't have the option to set mesh per device.
Or any other fix (besides buying an extra M9)