Welcome to the Auth0 Community.
I understand the key issue here is you are not able to verify the access token on the backend. In the first instance I would review our docs in this area i.e. our Express backend guide as it provides a walk through of how you would validate the access token https://auth0.com/docs/quickstart/backend/nodejs/01-authorization#validate-access-tokens
You may have already seen this given you mentioned the backend uses the respective SDK.If you had some issues in validating the access token can you elaborate on what those issues were?
Warm regards.