IOS generates a PEM certificate

OpenSSL pkcs12 -in certificates. P12 -out certificates. PEM -nodes

These files need to be converted to PEM format via terminal commands : OpenSSL pkcs12-clcerts-nokeys-out apns-dev-cert.pem-in APNS-DEV-CERT.P12

OpenSSL pkcs12-nocerts-out apns-dev-key.pem-in APNS-DEV-KEY.P12

If you want to remove the password, either do not set or execute it when exporting/converting:

OpenSSL rsa-in apns-dev-key.pem-out Apns-dev-key-noenc.pem

Finally, you need to synthesize the keys and license files into a APNS-DEV.PEM file, which you need to use when connecting to APNs:

Cat Apns-dev-cert.pem Apns-dev-key-noenc.pem > Apns-dev.pem