For any others who might run into this. In System Admin I kept getting " Error parsing /etc/pki/tls/certs/localhost.crt " no matter what certificate I was trying to install.
I found this https://issues.freepbx.org/browse/FREEPBX-10001 and used the command: /var/www/html/admin/modules/sysadmin/hooks/update-sslconf
which allowed me then to click 'install" on the commercial certificate I uploaded in SysAdmin and work properly.
Sysadmin Version 126.96.36.199
PBX Version: 188.8.131.52
PBX Distro: 12.7.6-1904-1.sng7
Asterisk Version: 13.22.0