Connection timed out - connect(2) for "api.dropbox.com" port 443

Dear gentlemen:

I got some troubles in using the Core API in Ruby.
All codes are written according to this document: https://www.dropbox.com/developers/core/start/ruby
as below:

def new_from_dropbox
  @uploads = Upload.order('created_at DESC')
  @upload = Upload.new
  # Install this the SDK with "gem install dropbox-sdk"
  require 'dropbox_sdk'
  # Get your app key and secret from the Dropbox developer website
  app_key = '***'
  app_secret = '***'
  flow = DropboxOAuth2FlowNoRedirect.new(app_key, app_secret)
  @authorize_url = flow.start()
end

def dropbox_confirm
  require 'dropbox_sdk'
  app_key = '***'
  app_secret = '***'
  flow = DropboxOAuth2FlowNoRedirect.new(app_key, app_secret)
  access_token = flow.finish(code_param[:code].strip)
  puts "================================"
  puts access_token.inspect
  client = DropboxClient.new(access_token)
  puts "linked account:", client.account_info().inspect
  root_metadata = client.metadata('/')
  puts "metadata:", root_metadata.inspect
end

private
def code_param
  params.require(:upload).permit(:code)
end

When i input the authorization code and the function dropbox_confirm try to get access_token by

access_token = flow.finish(code_param[:code].strip)

It returned an error:

Connection timed out - connect(2) for "api.dropbox.com" port 443

Anyone can help me? please.
I found some relative questions here: https://www.pythonanywhere.com/forums/topic/1806/
But that is for python.

Regards.