Your solution won't work until you add some logic for Sitecore / layout / Page that once there is this field on Page it needs to redirect.
Probably this would be a seperate component that will be part of standard values on your custom template.
I assume based on your description that you have currently only field where you specified External link but that's it.
There is a Redirect template that you can use next to standard Pages as you are using SXA.
A redirect is a way to send both users and search engines to a different URL from the one they originally requested. For example, to redirect visitors who enter namewebsite.com/home-a in their browser to namewebsite.com/home-b. This is very useful when you want to redirect a specific page to a new location, change the URL structure of a site so that it appears higher up in the navigation, or even redirect users to another website entirely. You can also use the mapping tool to set up a 301/302 or server transfer redirect.
To add a redirect item:
In the Content Editor, right-click on the page where you want to specify your redirect, and click Insert, Redirect.
Enter a name for the redirect item and click OK.
In the Redirect section, enter the URL that you want to direct to and save your changes.
Taken from Official Sitecore SXA Documentation:
It should exactly fulfill your requirements.