When installing an SSL certificate on a server you should install all intermediate certificates as well. Paste or upload your certificate to generate a .crt-file with all intermediate certificates concatenated.
All operating systems contain a set of default trusted root certificates.
But Certificate Authorities usually don’t use their root certificate to sign customer certificates.
They use so called intermediate certificates instead, because these can be rotated more frequently.
If not all intermediate certificates are installed on your server, some clients —mostly mobile browsers—
will think you are on an insecure connection.
More info and to test your certificate chain go to https://certificatechain.io/