Seeing a router's admin page when you visit the external IP of a network suggests that the router is configured to allow remote connections to it's administration interface.
You can prevent this by changing the router's settings so that it does not allow remote management. With remote management disabled, visiting the external address of a network should produce an error in the browser.
It isn't possible to provide specific instructions about how to disable remote management without knowing the specific model number of the router concerned however the setting is often labelled Remote Management or similar.