What exactly do you mean by “same clock”? Do both of these devices display the local time on their screen and the time is wrong? If so, what does the time on each device show (what is it plus or minus compared to the actual time)? Do you have the option to specify the timezone under Roon Settings -> Audio -> Cogwheel icon next to zone -> Device Setup -> Show Advanced?
With Ethernet the synchronization was perfect. And afterwards it remained fine with WiFi. Was rebooting the Flex the trick?
Anyhow: problem solved. Thanks…