Extend network with both wired and wireless base stations

Our house is a wifi black hole and I've tried to do what I can to get everything to work. I'm running on a new TC and extended the network by putting an Airport Extreme in the bedroom. I've connected them with an ethernet cable I ran in the crawlspace and they are both working perfectly. My problem is that there still isn't a good wi-fi signal in the bonus room. Running an ethernet cable upstairs isn't very practical so I got an Airport Express and tried to add it to the network wirelessly. I used the same steps I used to add the Airport Extreme but I get an error every time saying Airport Utility could not successfully be added to the network. Does anyone know if it's either one way or the other? Wired or wireless? Or is there possibly another issue? Computers and wireless devices work ok, but streaming on my AppleTV just doesn't seem to want to work. The content doesn't even have to be coming from the internet. I've tried steaming movies and TV shows saved on my Mac Mini, and it just doesn't do well. The Mac Mini is downstairs next to the TC and hooked up with ethernet, so I know it's not a wi-fi back and forth issue. I've even tried changing out the AppleTV but each one I put in the bonus room does the exact same thing. Any help or insights would be greatly appreciated!