I found the answer.
for those who may experience the same issue in the future I’ll leave a note here
when initiating auth0 login page in your app, it should point to custom DB that was set up in auth0 rather than default Username-Password-Authentication
var authorizationUrl = client.BuildAuthorizationUrl()
.WithResponseType(AuthorizationResponseType.Code)
.WithClient(clientId)
.WithConnection("YOUR-CUSTOM-DB-CONNECTION-NOT-Username-Password-Authentication")
.WithRedirectUrl(root + redirectUri)
.WithScope("openid offline_access profile email")
.Build();