If you don't understand the security risks around port forwarding then I suggest you get someone involved who does. For secure remote access you would be best to use a VPN. They are very simple to set up.
If your customer wants a cheap solution and doesn't care about ransom ware then port forwarding would be fine. As long as your customer knows the risks, bear in mind if you are advising the customer of solutions you are potentially liable if it all goes wrong.
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