Here’s how I just improved my wifi coverage with my existing Asus router.
I have a fairly large, 3000 sq foot, 1 ½ story house with a wifi dead zone that’s been bothering me for a bit. I have an Asus RT-AC68U router, and it actually does a good job for a relatively inexpensive router.
I noticed Asus has added a new feature called “Aimesh”, unintentionally waited a bit, probably a good thing to let Asus work out the bugs with early adopters lol. Decided to try a few days ago.
In a nutshell, Aimesh implements a mesh router environment, but you don't need to buy an expensive set of mesh routers, works with existing Asus routers, although not all of them.
To implement Aimesh, you need at least two compatible Asus routers. I already had a 2nd Asus router, a Tmobile RT-AC1900, which is a rebranded RT-AC68.
So with my two Asus routers, I’m in business with no extra costs. I did need to load the Tmobile router with AC68 firmware, fortunately there are good instructions out there to do this, but this turned out to be the most work. Took a couple of days, and part of the problem was I forgot to do the last step

So once I had my 2nd router running with the latest Asus 68U firmware, I followed these instructions
https://www.asus.com/support/FAQ/1035087 And it worked!!
Then I moved the 2nd router (Aimesh node) to its new home. Its actually pretty close to the main Aimesh router, but on the other side of a few walls. Now I have good, fast coverage where I had marginal to bad coverage before. Its seamless, devices will automatically switch to the router with the best wifi signal as you walk around. And if you have an existing, compatible Asus router already, you can pick up a 2nd one fairly cheaply to do this.
I did read for the node router(s), it works better if you connect it via ethernet to the main Aimesh router, instead of using wifi. I’m using ethernet, and it seems to work well. I imagine if you had to connect wifi, it would still be better than not having it, but probably not as good as ethernet.
Good luck if you give it a go.
Randy