you can test your cert install against an online sslChecker:
It will tell you immediately if something is wrong with your CA cert or immediate cert.
but the SSL Certificate seems to be untrusted. What are the possible reasons for that?
Most likely, your common name assigned at certificate signing request doesn't match the hostname you're using the cert with. You need to make sure these names match, domain and subdomain. The only way around using the same cert on multiple subdomains, is to use a wildcard cert. However, this is typically a different service than the traditional cert services, single domain certificates.
Apache CSR Example with openssl:
$ openssl req -new -newkey rsa:2048 -nodes -keyout myPrivatekey.key -out mycsr.csr -config openssl.conf
Generating a 2048 bit RSA private key
writing new private key to 'mykey.key'
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
Country Name (2 letter code) [AU]:US
State or Province Name (full name) [Some-State]:North Carolina
Locality Name (eg, city) :Raleigh
Organization Name (eg, company) [Internet Widgits Pty Ltd]:Somebody,
Organizational Unit Name (eg, section) :IT
Common Name (eg, YOUR name) :test.testdomain.com (MUST MATCH HOST)
Email Address :firstname.lastname@example.org
Then you would send it off to your CA provider and they would generate the public, root, and intermediate certificates. Use your private key generated with the CSR.
Apache httpd.conf settings:
# Secure (SSL/TLS) connections
#CA certificates for root and intermediate
#Generated first via openssl; Server public and private keys.