Webmin - add SSL certificate


I bought SSL certificate and am trying put this into webserver
I m using nextcloud-based-in solution by webmin.
i replaced ssl certificate on webserver (etc/ssl - *.pem and *.key)
after then i restarted apache and nextcloud website is unavailable.

I don´t understand what is wrong, or what i need to do, for correct working ssl…
(If i turn back the old ssl certificate, all work fine, but webpage is unsafety.)

please help, or some guide, how to correctly apply the ssl certificate on webserver.