Draytek Vigor3910 Series User Manual page 93

For example, the built-in web user interface in the router is with default port 80, which may
conflict with the web server in the local network, http://192.168.1.13:80. Therefore, you
need to change the router's http port to any one other than the default port 80 to avoid
conflict, such as 8080. This can be set in the System Maintenance >>Management Setup. You
then will access the admin screen of by suffixing the IP address with 8080, e.g.,
http://192.168.1.1:8080 instead of port 80.
Vigor3910 Series User's Guide 83