If you’re using a mobile phone, try switching between mobile data and Wi-Fi connections. Sometimes one connection type works when another does not. If you’re using a computer, try disconnecting from broadband and reconnecting.
If you continue to experience difficulties, there is likely a problem with your Outline server. Contact your service manager for a new access key for an alternative server. They may also be able to increase your access key’s data limit if the key has exceeded its quota.