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 the plus icon to create a new certificate
Select a new “Apple Push Notifications service SSL (Sandbox & Production)”
Choose the App ID you’re generating a certificate for. Use the regular App ID for the Mattermost app, not for the
Choose the certificate request file created using the Keychain access in the previous section and select 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.