You need to have a service of some sort running on the callback URL that is listening for HTTP GET and POST requests. That’s how webhooks perform the initial subscription process and then when sending notifications.
If you’re just using Client ID and not a Bearer token in the request, the ratelimit is 30, not 120. If you’re still hitting the rate limit you must be doing more than 30 requests a minute. Also, requesting any API point that frequently is excessive as all endpoints use caching, so you will be getting back the same cached data needlessly.