Server certificates on Android question

Hello everyone. So I built my apk (fully working in the editor with server as well) and I got the following error. I already checked manifest for internet permission.

javax.net.ssl.SSLHandshakeException: java.security.cert.CertPathValidatorException: Trust anchor for certification path not found.

This happens when I try to communicate with my server through both WWW and UnityWebRequest functions. Checked with both “http” and “https”. So, I found out it’s a self-signed / missing intermediate certificate issue.
I checked on DigiCert and found out my server has indeed untrusted certificates :
124297-debug2.png
I decided the to install openssl plugin and test some more, so i run the following line in cmd :

openssl s_client -debug -connect www.thedomaintocheck.com:443

and the logs that follows are some that I think are important. (i can give the full log if needed)

......
depth=0 C = US, ST = Washington, L = Seattle, O = Odin, OU = Plesk, CN = Plesk, emailAddress = info@plesk.com
verify error:num=18:self signed certificate
verify return:1
depth=0 C = US, ST = Washington, L = Seattle, O = Odin, OU = Plesk, CN = Plesk, emailAddress = info@plesk.com
verify error:num=10:certificate has expired
notAfter=May 20 07:48:11 2017 GMT
verify return:1
depth=0 C = US, ST = Washington, L = Seattle, O = Odin, OU = Plesk, CN = Plesk, emailAddress = info@plesk.com
notAfter=May 20 07:48:11 2017 GMT
verify return:1
......

and

......
    Certificate chain
     0 s:/C=US/ST=Washington/L=Seattle/O=Odin/OU=Plesk/CN=Plesk/emailAddress=info@plesk.com
       i:/C=US/ST=Washington/L=Seattle/O=Odin/OU=Plesk/CN=Plesk/emailAddress=info@plesk.com
    ---
    Server certificate
    -----BEGIN CERTIFICATE-----
    // stuff
    -----END CERTIFICATE-----
    subject=/C=US/ST=Washington/L=Seattle/O=Odin/OU=Plesk/CN=Plesk/emailAddress=info@plesk.com
    issuer=/C=US/ST=Washington/L=Seattle/O=Odin/OU=Plesk/CN=Plesk/emailAddress=info@plesk.com
    ---
    No client certificate CA names sent
    Peer signing digest: SHA512
    Server Temp Key: ECDH, P-256, 256 bits
    ---
    SSL handshake has read 1567 bytes and written 302 bytes
    Verification error: certificate has expired
    ......

and

......
Start Time: 1536579311
Timeout   : 7200 (sec)
Verify return code: 10 (certificate has expired)
Extended master secret: no
......

It’s weird cause I get expired certificate while the site before game me the following results…


But I also get a self-signed certificate, so which one is causing to fail?
So, how should I go on from this? should I reach my server provider with this data?
Or maybe i have a way to install the intermediate certificates said above? But how do I know which ones?
Thanks for your time!

Contacting the server provider with the same data posted here, helped them solving the problem for me.