Push notifications on iOS are managed and dispatched using Apple’s Push Notification Service. You must have a Paid Apple Developer account to create certificates needed to send notifications using this service.
Generate a Certificate from Keychain Access
Launch the Keychain Access application in your Mac and select KeyChain Access -\Certificate Assistant -\Request a Certificate From a Certificate Authority…
Enter your email address in User Email Address and check the “Save to disk” option, then click Continue
Save the certificate request
Log in to Apple developer account and click Certificates, Identifiers and Profiles
Select iOS from the dropdown
Select App IDs from the side menu and look for the Bundle Identifier you are using for the Mattermost app
Select the App ID and click Edit
Scroll down to the Push Notification Section and click Create a Production SSL Certificate
In the About Creating a Certificate Signing Request (CSR) screen click Continue /img/ios_csr.png)
Choose the certificate request file created using the Keychain access in the previous section and click Continue
Download the Certificate and click Done to finish the process
At this point, you can build the Mattermost app for iOS and use the above downloaded certificate to setup the Mattermost Push Notification Service.