Facebook SDK - Get details of a lead - ASP .net core API

Question

What i try to do is retrieve my facebook leads from my facebook page in a webhook ASP.net core API , get the lead details and save it in my mongoDB Database.

I had a problem getting my lead details in the API because of the token.

And then i saw in the facebook developer docs : a. Get a regular user token. b. Convert it into a long-lived token c. With the long-lived user's access token, request the page token [..] This long-lived page token has no expiration date; you can hard-code it in simple RTU integrations to get leads data.

The problem is i got that page token like they show in their docs , but it expired one hour after i got it. But they say here it's not expiring. So i don't understand how should i do.

What i did is getting the token from the web , saving it in my database and when a lead comes search it in my database to do the request to get the lead details of the page.

I tried with a app token but i had an error that i need a user token but getting a user token from an API doesn't seems correct.

Tell me if i miss something. Really need your help here ! Thanks !

I have my Angular web and my ASP.net core API . I tried with a app token , and a page token gotten from the web.

Web :

///Login

FB.login(response => {
  if (response.authResponse) {
    this.userId = response.authResponse.userID;
    FB.api('me/accounts', responsePages => {
      this.ngzone.run(() => {
        this.pages = responsePages.data.map(elem => {
          let page = new FacebookPage();
          page.access_token = elem.access_token;
          page.category = elem.category;
          page.id = elem.id;
          page.name = elem.name;
          FB.api(page.id + "/subscribed_apps?access_token=" + page.access_token, res => {
            if (res.data.length > 0 && res.data[0].name === facebookAPPName && res.data[0].id === facebookAPPId)
              page.connected = true;
            else page.connected = false;
          })
          return page;
        });    
      });
    })
  }
  else {
    console.log('User login failed');
  }
}, { scope: ['manage_pages', 'leads_retrieval'] });

///Link Page

FB.api(this.userId+"/accounts", res => {
      let accessToken = res.data.filter(elem => elem.name === page.name[0].access_token;
       FB.api('/' + page.id + '/subscribed_apps',
      'post',
      { access_token: page.access_token, subscribed_fields: ['leadgen'] },
       (response) => {
        console.log('Successfuly subscribed ' + JSON.stringify(response))
        page.connected = true;
        page.access_token = accessToken;
        this.facebookService.link(page).subscribe(()=>{
                  this.spinner.hide();
        },
        err => { console.log(err); this.spinner.hide();})
      })
    })

API :

[HttpPost]
        public IActionResult Post([FromBody]FacebookLead data)
        {
            var entry = data.entry.FirstOrDefault();
            var change = entry?.changes.FirstOrDefault();

            var leadId = change.value.leadgen_id;

            var client = new HttpClient();

            var page = this.facebookService.Get(change.value.page_id);

            var request = client.GetAsync("https://graph.facebook.com/v3.3/" + leadId + "?access_token=" + page.access_token);

            request.Wait();

            var res = request.Result.Content.ReadAsStringAsync();

            res.Wait();

            var result = res.Result;

            return Ok();
        }

Expected output is the detail of the lead , without expiration of token in API. But i get an error either because the token expired or because app token doesn't work