https://realm.io/docs/realm-object-server/latest/#extending-functionality
Here I see what appear to be instructions for extending the ROS to be a web service. Server side access is called a pro feature in the pricing page https://realm.io/pricing#price-cards so are those functions only available in the pro edition?
No - those features are available in the developer edition. What is referred to as server-side access concerns writing server-side apps that use synchronized realms outside of ROS.