GetDiscoveryDocumentAsync can not find the authority url - IdentiyServer4 - asp.net-core

Question

I was trying to replace obsolete IdentityServer methods and types and as I had been warned about, I tried to replace DiscoveryClient() and TokenClient() with appropiate methods like in the examples of the latest identity server docs. When I try to get related endpoints by GetDiscoveryDocumentAsync it returns null even though I could read those values with current code and also get those values on browser.
Besides, when I by-pass the step of discovery and supplying the direct token endpoint RequestTokenAsync returns null because of Not Found exception.
For the sake of clearity of the question I should say that I have not changed anything (its settings or endpoints) in my Identity server project (from which I try to get access token).
Followings are my previous and updated code to achieve what I've described. Any help or suggestion is appreciated. Thanks in advance.
Previous Code (Working):
var testServer = new TestServer(builder);
var client = testServer.CreateClient();
client.BaseAddress = new Uri("http://localhost:5000");
var discoClient = new DiscoveryClient(AuthorityUrl) {Policy = {RequireHttps = false}};
var disco = discoClient.GetAsync().Result;
var tokenClient = new TokenClient(disco.TokenEndpoint, ClientId, ClientSecret);
var tokenResponse = tokenClient.RequestClientCredentialsAsync(Scope).Result;
client.SetBearerToken(tokenResponse.AccessToken);
Updated Code (Not Working):
var testServer = new TestServer(builder);
var client = testServer.CreateClient();
client.BaseAddress = new Uri("http://localhost:5000");
//var discoClient = new DiscoveryClient(AuthorityUrl) {Policy = {RequireHttps = false}};
var disco = client.GetDiscoveryDocumentAsync(new DiscoveryDocumentRequest()
{ Address = AuthorityUrl, Policy = new DiscoveryPolicy() { RequireHttps = false, Authority = AuthorityUrl} }).Result;
;
if (disco.IsError)
{
throw new Exception(disco.Error);
}
//var tokenClient = new TokenClient(disco.TokenEndpoint, ClientId, ClientSecret);
var tokenClient = client.RequestTokenAsync(new TokenRequest()
{ Address = disco.TokenEndpoint, ClientId = ClientId, ClientSecret = ClientSecret, GrantType = GrantType}).Result;
//var tokenResponse = tokenClient.RequestClientCredentialsAsync(Scope).Result;
client.SetBearerToken(tokenClient.AccessToken);
return client;
Edit:
Updated my code as shown below and still getting the same error.
var testServer = new TestServer(builder);
var client = testServer.CreateClient();
client.BaseAddress = new Uri("http://localhost:5000");
//var discoClient = new DiscoveryClient(AuthorityUrl) {Policy = {RequireHttps = false}};
var disco = await client.GetDiscoveryDocumentAsync(new DiscoveryDocumentRequest()
{ Address = AuthorityUrl, Policy = new DiscoveryPolicy() { RequireHttps = false, Authority = AuthorityUrl } });
;
if (disco.IsError)
{
throw new Exception(disco.Error);
}
//var tokenClient = new TokenClient(disco.TokenEndpoint, ClientId, ClientSecret);
var tokenClient =await client.RequestTokenAsync(new ClientCredentialsTokenRequest()
{ Address = disco.TokenEndpoint, ClientId = ClientId, ClientSecret = ClientSecret, GrantType = GrantType , Scope = Scope});
client.SetBearerToken(tokenClient.AccessToken);
return client;
Error:
"Error connecting to AuthorityUrl/.well-known/openid-configuration: Not Found"