Hi: I have one public IP and I have my old webserver and the new one with ISPConfig. How can I do for beeing able to display pages from both servers. I have tried with proxy directives but it doesn't work. Could you help me please ?
You can do this by using proxy directives. Point the router IP to the first server and then create a website there which uses proxy to forward the requests to the second server.
Another alternative would be a web proxy on your router that knows where to send requests for a given domain. (Eg. pfsense can do that.)