Very Respectfully, Merlin, I dont believe that is entirely correct and I will explain why in a few sentences down. To the op, if you want DFS on an ac68u, you will need to flash an older Merlin based firmware such as johns fork OR an alternative firmware like dd-wrt or fresh-tomato, both of which have modern (as of march 2021) firmwares with modern(late 2020 or better drivers) that still have enabled the DFS channels. I know this because I have this router (actually 4 of the ac68u) and use all 4 mentioned firmwares as a hobby to play with settings and compare the differences between firmwares, mess with scripts etc so I know this to be true. it's only the asus stock firmware and merlins firmwares that are more recent then johns fork that frustratingly, have the DFS channels locked down and not available.
I dont know why this is because DFS is legally allowed in my country (USA) as well as a host of other nations.
Ive seen others suggest that the more recent Broadcom drivers have DFS disabled and maybe thats why and I have no arguement with that other then to say that Fresh-Tomato and dd-wrt use broadcom drivers less then a few months old and they support DFS....