You’re getting an opaque access token most likely because you’re not including an audience in the authorize request. You can configure a custom API in the Auth0 dashboard and then use its identifier in the authorize request, as the audience.
You can try changing your Auth URL to something like:
https://{{auth0Domain}}/authorize?audience=YOUR_API_IDENTIFIER
You can also read more about opaque and JWT access tokens here:
https://auth0.com/docs/tokens/access-tokens/get-access-tokens#control-access-token-audience