How to resolve File Not Found in SharePoint 2013?

Dear EE experts,

We would like to ask for tech support on how to resolve the "File Not Found" error in our SharePoint 2013.
Here's the case, every time we access our SP site locally, it means if we access it using this site address: http://pm-server:2018/products/Gallery/Forms/Thumbnails.aspx
everything's fine, every pages are working properly and smoothly. But when we access this site using this address: http://58.137.29.120:2018/products/Gallery/Forms/Thumbnails.aspx    (that site address is for our external use), we encounter "File Not Found", "Sorry, something went wrong. File Not Found."

But if we access the site (for example, the Gallery) using the local address, of course it will show because it has no problem, then we go back to our external site, then access the Gallery again, or refresh the page, it will show no problem anymore... But how about to our clients who needs to access the external site, they can't fix it or they have no idea how to fix the problem that's why we must have the solution for this... and that's why we're asking for experts advise... :)

Thank you and hope to hear soon...