It could be compatibility issue. You may try to connect a repeater with different model to the MR3420 and also try update the firmware for the repeater.

For the NAT, it is enabled by default as long as you use this as a router, i.e. get internet from 3G/4G modem or by connecting your ISP to the WAN port. About the reboot, mostly it is needed. The...

What's the model of your phone and the model number of your product? If yours is the same as nevilled's, you may try the beta: http://static.tp-link.com/MR6400V1(EU)_170411.zip

I don't think there is one. The only thing I got is the URL for video streaming: 720P: rtsp://admin:Password@IP/h264_hd.sdp 480P: rtsp://admin:Password@IP/h264_vga.sdp

I don't think there is one. The only thing I got is the URL for video streaming: 720P: rtsp://admin:Password@IP/h264_hd.sdp 480P: rtsp://admin:Password@IP/h264_vga.sdp

Can also try the powerline scan utility in this link: http://www.tp-link.com/en/download/TL-WPA4220_V1.html#Utility It can also lead you to the web interface to change the wifi settings.