For the last couple of days I have been struggling with this myself. Eventually found the answer on GitHub - jlhughes/openwrt-yamon: YAMon2 for openwrt even though the link is from 2015.

It seems as if you only need to change the links to make it work.

Do this via Putty:
cd /www
ln -s /tmp/www yamon

Then this link should work;

http://router_name/yamon/index.html