@3t_ed usually 2.4Ghz has more range, while 5Ghz is faster.
But that was not my point.
In my router, I can configure if I want to have 2 seperate wifi-accespoints for each frequency, or if I want to have only one accespoint and the devices and/or the router are deciding dynamically which frequency will be used.
My impression is, that the issues described by jimpson, are happening if my router is configured to only provide one accesspoint for both frequencies.