Self-Signed Cert with TLS 1.2

As a workaround you could add a handler to the ServicePointManager's ServerCertificateValidationCallback on the client side:

System.Net.ServicePointManager.ServerCertificateValidationCallback +=
    (se, cert, chain, sslerror) =>
        {
            return true;
        };

but be aware that this is not a good practice as it completely ignores the server certificate and tells the service point manager that whatever certificate is fine which can seriously compromise client security. You could refine this and do some custom checking (for certificate name, hash etc). at least you can circumvent problems during development when using test certificates.

You can create a non self-signed certificate in development area and then use this certificate in IIS for applying the SSL. The steps are:

1. Create self-signed certificate

   makecert -r -pe -n "CN=My Root Authority" -a sha1 -sky signature 
       -ss CA -sr CurrentUser  
       -cy authority 
       -sv CA.pvk CA.cer

2. Create a non self-signed certificate for SSL which signed by this root certificate and then create pfx-file from that

   makecert -pe -n "CN=servername" -a sha1 -sky exchange
       -eku 1.3.6.1.5.5.7.3.1 -ic CA.cer -iv CA.pvk
       -sp "Microsoft RSA SChannel Cryptographic Provider"
       -sy 12 -sv server.pvk server.cer
   
   pvk2pfx -pvk server.pvk -spc server.cer -pfx server.pfx

now you just need to import the server.pfx into the IIS and setup the web site binding to use this certificate and also install the CA.cer in Local Computer \ Trusted Root Certification Authorities store in both server and client by doing this WCF client would work with the service through HTTPS without any problem.