Hello my friend!
Follow the documentation abou GXP Series.
http://www.grandstream.com/products/gxp_series/general/documents/gxp21xx_usermanual_english.pdf
See the page 35, you must specify a directory to the SSL certificate, to use TLS.
- SSL Certificate - The user specified SSL certificate used for SIP over TLS.
- SSL Private Key - The user specified SSL private key used for SIP over TLS.
- SSL Private Key Password - User specified password to protect the private key above.
Tks!