The access token and ID token are not the same thing. Your backend isn’t going to accept the ID token.
You need to pass an audience param with the request to get a JWT that your backend can consume, otherwise you are going to get an opaque token.
You can see how to add an audience param in this guide: Auth0 Angular SDK Quickstarts: Call an API
Alternatively, you can add it to the request to getAccessTokenSilently.
Hope this helps!