IP Address Changed
This dialog appears when it is detected that your public IP address has changed. It is only possble to detect this when UPnP is enabled on the router and Options -> Routing -> From Public Internet is set to "Static IP (not sure)".
That there has been a change indicates that you have a dynamically assigned IP address from your ISP. There are three options here: tell your users about each change; request a fixed IP address from your ISP (this usually costs extra); or use a dynamic domain name service.
Informing your users each time the address changes is very labour intensive and prone to error. The easiest solution is to request a fixed IP address from your ISP. Alternatively you can make use of a Dynamic DNS service to provide a mapping between a domain name and your changing IP address. There are several Dynamic DNS services that provide free service (there may be some limitations) or a fee paying service can be used together with your own domain name. Some providers are listed on this page.