Using my ASUS RT-AC66U to improve WiFi

[Crickets]

Hey guys, first of all im not a native english speaker so please excuse my english skills.

My problem: I want to use my ASUS RT-AC66U to improve the WiFi in my flat. I already have another router set up and set the ASUS router to repeater mode. The Problem is, from time to time there is no internet with the ASUS router but i can still connect to the other router.
Im not that advanced in those things but i understand the basics. Anyone has an idea how to fix my problem?

 

[joegreat]

My first suggestion would be to update both routers to the latest firmware version - hopefully both routers are Asus brand and you can use Merlin's fork!?

Do a factory reset after the update and reconfigure the routers manually - than test if things work as you expect it!