You cannot use the router as a web server like that.
If you need it to be done on the same device your best bet is a metarouter or KVM virtual machine that in turn runs a web server. That will, however, take quite some resources. Why does the destination IP address of your web server change regularly? Can you use dynDNS or changeIP or such a service to automatically update the DNS records on IP change so you don’t need to change the proxy configuration?
It doesnt change so often, but I asked support and told me that the IP can change if the have to move my hosting to another server. This things happend sometimes without the user even notice.
I don’t know where to use dyndns or noip.org if the dst address IP have to be enter manually on the webproxy config.