The first thing is: are both routers the same frequency? You can't have a 2.4ghz signal trying to connect to a 5ghz signal, in either direction.
I can also say that the first time I set up mine I kept forgetting to click Save. So the settings didn't save when I went to another screen.
Go into the router's firmware. Click Wireless and it should be on Basic setup. This is where you make the bridge to your main router.
Wireless Mode = Repeater
Wireless Mode = Mixed (because that will cover you no matter what the main router is)
Wireless Network Name (SSID) = whatever your main router's SSID is
Click Save down at the bottom of the screen
Just below where you entered that information is the heading: Virtual Interfaces with the a button labeled Add, click Add
Just type the same SSID that you did above and click Save.
So what you have done is told the repeater that it's going to connect to the main router just like any other device would but you need to give it the password of the wireless, just like you do every device. When you want to connect you click on the SSID and then Join and it normally asks you for a password. You enter it and click Connect. It authenticates you and gets you an IP and bam you're connected.
Now we have to tell it what the password is so it can connect to the main router.
Click on Wireless Security
The first one is the Physical(the one that bridges the connection) - this is like any other device connecting.
Security Mode = whatever it is on the main router normally WPA2 Personal
WPA Algorithms = whatever it is on the main router - TKIP or AES or both - when in doubt click the one with them both
WPA Shared key = whatever it is on the main router
Then for the Virtual interface you'll want to enter the same things and click Save.
Making sure you type the SSIDs the same and the passwords the same is REALLY important and clicking SAVE. But you have to make sure they are the same frequency.