This update was just released yesterday. Numerous Asus routers are supported. This was a feature that was also found and slowly being implemented into Tomato. Now it's built into the Merlin firmware by default.
The great thing about this is that it enhances and improves security overall, and it far more secure than DNS over HTTPS. I strongly recommend making use of this feature.
Read more about it here: https://github.com/RMerl/asuswrt-merlin/wiki/DNS-Privacy
Downloads: https://asuswrt.lostrealm.ca/download