LetsEncrypt requires a web server, typically either Apache HTTPD or IIS, to confirm the server's identity. The name of your certificate files suggests you are using Apache. In this case, follow instructions here to convert the Apache cert into a format that is used by Xeams. Once the cert is converted, you should be able to use that in Xeams.

One downside of using LetsEncrypt is that you will have to repeat this process every 3 months.