TCP connection 'freezes', UDP too slow (Synology)

[filarakia 1]

Hi everyone,
I have been using AirVPN on my Synology (on BT UK) for many months, always connecting to the EU servers on TCP, and it works perfectly. Transfer speeds are always phenomenal (about 3 to 4MB/s, 80%-ish of my actual connection speed).
For the past few days however, this has changed. If I connect via TCP, the transfers freeze after a while, could be a minute or could be half an hour. All I can do is disconnect/reconnect. Today, it freezes every single time. I can connect via UDP, and the connection is stable but extremely slow (400KB/s).
Are there any known issues that could be causing this?
I know that theoretically UDP is supposed to be faster/better/more stable than TCP, and I (and others) have never been able to explain how I get such good speeds over TCP, but there you go. Any ideas are welcome.