Convert your ssl certificates to pem format using following commands
openssl pkcs12 -in yourP12File.pfx -nocerts -out privateKey.pem
openssl pkcs12 -in yourP12File.pfx -clcerts -nokeys -out publicCert.pem
Then run following command to ensure that there is no issue with your certificate or network connection.
openssl s_client -connect gateway.sandbox.push.apple.com:2195 -cert YourSSLCertAndPrivateKey.pem -debug -showcerts -CAfile "Entrust.net Certification Authority (2048).pem"
You should download Entrust certificate and convert it to pem as APNS certificates are signed by Entrust.