RT-AC68U can no longer be overclocked by nvram, which is a feature only available in firmware 378 and older. If you use nvram overclocking, the router firmware will ignore your changes.
For RT-AC68U shipped after 2019, they have a brand new CFE, so you can’t use the CFE command to overclock. This will only bring about 10% of the CPU.
However, you can use CFE that can be overclocked through CFE downgrade, but BCM4709 cannot use BCM4708 CFE, you need to be especially careful about this.
BTW, the CFE command is very dangerous, because it will modify your CFE, and you cannot restore the changes you made by resetting. If your frequency was 800 before and you change it to 1000, this will be an extra character. So guess what will happen?
Edit: the above information has not been verified by asus or any developer, it is just my experience, and there may be not exactly right.