Hi @Terry Chan ,
You could try to use AAM to configure the full URL. Alternate Access mappings in SharePoint allows you to create multiple URLs for the same web application. This will helps users from the internal company network users to access the intranet site by simply using http / https URL and forcing outside the network users accessing it from the internet to use the fully-qualified domain name.
Here are some blogs about how to cofigure AAM step by step you could have a look:
Configure alternate access mappings SharePoint 2016 step by step
SHAREPOINT 2016: HOW TO IMPLEMENT ALTERNATE ACCESS MAPPING
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Reference:
Configure alternate access mappings for SharePoint Server
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