That's pretty strange! Let's Encrypt certificates should work as well in Malaysia as anywhere.
Can you share the domain name(s) that the certificate was issued for?
Can the client provide more information about how it failed for them? Ideally a screenshot of the error & if possible the certificate that was presented to the browser. It would also be helpful to know more about the browser and client operating system versions.
Usually this kind of thing is caused by a missing intermediate in the server configuration or a hostname mismatch. Occasionally it can be caused by an ISP or piece of networking gear doing traffic inspection.