RFC1918 error indicates, that you are trying to reach router's Public IP address from a device, which is already connected to RUTX device locally.
If you would try to reach same IP address from any other device (e.g. from mobile phone via mobile internet) you should be able to reach router's WebUI.
Or course, you can disable RFC protection. That can be done througt SSH:
in /etc/config/uhttpd , put at 0 the rfc1918_filter.