At 22:18 we had a large number of data SIMs drop their connection. They are reconnecting at the moment, but it looks like some are struggling with their connection. This looks like an upstream problem - ie within the mobile network or the network operator.
Data SIMs are now stable.
Just to provide more clarification on this, our carrier's network team was carrying out works on one of their routers. This was not expected to affect Data SIM routing, but had an unforeseen knock-on. They've identified the root cause and are taking steps to rectify this.