Restrict users from premium features.

It depends on how secure you want to make it.

For maximum security, all actions need to happen on the backend, and there you can check the user's permissions. Usually, you create one or more permission groups on the backend, and then assign the user to one or more groups, depending on their subscription plan (different plans hold different quotas). You can send these permissions to the frontend, to hide some functionality/pages, but ultimately, you need to check for every action on the backend, if the user has permissions and quota hasn't been exceeded yet for the given time period (check user's quotas against subscription plan quotas and run a cron job that resets users quotas at the end of the month) .

You will need to look into some Role Based Authorization. Something like this may help https://jasonwatmore.com/post/2018/11/22/angular-7-role-based-authorization-tutorial-with-example

What will you using be using as your backend?

You need to have this logic live on the back-end, anything that's front-end only is insecure. My way to do this would be to have some sort of middleware that sits in front of the routes that handles doing auth checks. Each time the protected route is hit the middleware gets triggered, and either allows the action or sends back some sort of error if the user has exceeded their privileges. How you set that up depends on your back-end and any frameworks you're using.

This can be sorted via conditional statements.