How to use an api that is mapped to a service in Kubernetes
Your service seems to be ok, the issue could be possibly because your frontend is not server rendered, which means that your browser is trying to lookup for a name backend, in that case you cannot rely on kubernetes service name as your browser does not recognize it as a valid hostname.
If you want to access externally by instead of ip, you want to use names, check how to setup a ingress entry https://kubernetes.io/docs/concepts/services-networking/ingress/#what-is-ingress