403 means the server you are trying to talk with is refusing your request. If you have control of this server, there should hopefully be some logging there explaining what happened.
If you don’t have control of the other server, then I would turn up the logging on REST Consume lognode in your Mendix application to TRACE. That will show you the request and response headers being sent by the app. Are you seeing the Bearer token in the request headers?
if this doesn’t help, can you share your microflow that is making the REST call, and in particular the Request tab of the REST call action.