Created an API but getting 511 network authentication error at client end - rest

Question

I am trying to create a simple web app, where I am using Go to create the backend and svelte to create the frontend. Backend is hosted on port 8080 and frontend on 5173. I used a node package 'localtunnel' to forward both the ports.
When I am sending a GET request on the forwarded backend port it is working fine but when I am using fetch(window.fetch) on the same from the svelte app it is giving 511 network authentication error. Please see the below codes:
Go Backend:
func enableCors(w *http.ResponseWriter) {
(*w).Header().Set("Access-Control-Allow-Origin", "https://chilly-rabbits-film-146-196-44-202.loca.lt")
//^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ I am providing the forwarded address of svelte app (PORT: 5173)
}
// Wrapper handler function
func handleRequest() {
handler := mux.NewRouter()
handler.HandleFunc("/sant", argHandler).Methods("GET")
log.Fatal(http.ListenAndServe(":8080", handler))
}
func argHandler(w http.ResponseWriter, r *http.Request) {
enableCors(&w)
w.Header().Set("content-type", "application/json")
.
.
.
var data Data
_ = json.Unmarshal(file, &data)
var sant string
sant = <some data is assigned>
json.NewEncoder(w).Encode(santa)
Svelte frontend:
<script>
let name;
async function fetchData() {
const endpoint = "https://giant-words-help-146-196-44-202.loca.lt/sant";
//^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ forwarded port of Backend API(PORT: 8080)
const response = await fetch(endpoint, {
"Content-Type": "application/json",
Accept: "application/json",
});
name = await response.json();
}
</script>
Getting the following error:
GEThttps://giant-words-help-146-196-44-202.loca.lt/sant [HTTP/2 511 Network Authentication Required 1560ms]
If I open https://giant-words-help-146-196-44-202.loca.lt/sant once on a new browser tab then the whole thing some how starts to work. But for Each network who is accessing the app need to open the link once to access the app.
Could you please help me figuring this out.
Thanks in advance! :)