You will need to allow the below 5 URLs to be reachable from your Enterprise Alert Server in order to register your Signl4 teams in Enterprise Alert:
https://api.signl4.com/api/teams/
https://api.signl4.com/api/alerts/
https://api.signl4.com/api/v2/alerts
https://api.signl4.com/api/v2/eaRemoteActions