You have to open the port 8080 for the second WR941ND's WAN IP in first WR941ND's webpage. To access it remotely, you need to use http://WAN IP: port.

Hard Reset won't help. Do you know openwrt viods your warranty?

To try different channels, you should change channel for the root router and reconfigure WR941. What's the IP address of your root router and WR941? When disconnects, connecting to WR941, what's the...

Not sure why they display different from each other. What's the mode on your router's 5GHz page? If the router and PC are next to each other and signal strength is full, how about the speed displayed?

Can you browse webpages normally on that phone? Any other phones or tablets for you to test? Tried to change channel or channel width on the router?

1. Check it is a compabile drive or not. http://www.tp-link.com/en/article/?faqid=603 2. USB settings in new firmware is a little bit different from old version. But if it is a compatible USB driver,...