To get a mesh (wireless uplink) to work best, you place the second router, not where you have poor signal, but 1/2 way between the base router and the poor signal area.
I believe most newer Asus routers will Airlink together.
If you can get a wired connection to the second router, you will have much better performance. Do you have coax cable in the house ? That can be used for networking with MoCA adapters.