Hi @Alex_Campbell ---- Thank you for the report and sharing your feedback with us. The insight is appreciated and sorry to hear of the troubles.
Moving forward, can you please confirm if during your troubleshooting of this behavior you have tried rebooting not only the MBP but any relevant networking hardware as well, say for example, your router?
Additionally, does the MBP have any active firewalls or antivirus application currently running on it? Are you by chance running “little snitch”
-Eric