http://www.msftncsi.com/ncsi.txt
You can run a server on the device (you can use httpd of busybox) and redirect via the hosts file
www.msftncsi.com to your address and provide the ncsi.txt file there. The content of the file is
EDIT: Rereading your post I get that you use your router for the job. So my note on using a server on the device has no point.