Hi,
As I posted elsewhere - most of consumer devices don’t support 802.11r so there’s big chance it won’t help you even if you’ll worked it out. In similar (Czech) thread recently - Druhý router jako extender - was multiple workarounds suggested and OP successfully tried to set the same channels on both APs (together with same channel width and other radio settings) which worked for him but I would avoid such practice if there’s other solution.