ok, this was more easy then expected.
You only have to check and (if neccessary) add the missing parameters for each "config wifi-device" in "/etc/config/wireless" to get all bands working! yay
Have special attention to the "option band" parameter.
On one router the "option band '5g'" and on another the "option band '6g'" parameter was missing.
After adding them, all three bands started working
Here is a example of my router:
config wifi-device 'radio0'
option type 'mac80211'
option path 'soc/11300000.pcie/pci0000:00/0000:00:00.0/0000:01:00.0'
option band '2g'
option channel 'auto'
option htmode 'HE40'
option txpower '20'
option country 'DE'
option cell_density '0'
config wifi-device 'radio1'
option type 'mac80211'
option path 'soc/11300000.pcie/pci0000:00/0000:00:00.0/0000:01:00.0+1'
option band '5g'
option channel 'auto'
option htmode 'HE160'
option country 'DE'
option cell_density '0'
option txpower '23'
config wifi-device 'radio2'
option type 'mac80211'
option path 'soc/11300000.pcie/pci0000:00/0000:00:00.0/0000:01:00.0+2'
option band '6g'
option channel 'auto'
option htmode 'HE160'
option country 'DE'
option cell_density '0'