Thanks - I hadn't changed Time Zone recently (ie since installation): a clock change (to BST=GMT+1) _due_ to the Time Zone occurred in the UK at 0100 on 27/3 when the modem was switched off. The data reset date was also 27th month. When it switched on at 6 am on the 27th I got a warning text message and nothing more until today. (Text message commands still worked - there was clearly still a strong 4g signal.)
The modem has started communicating again today after 3 days of data silence. I've turned off the data limit - I don't trust it and I can't have this happening again.
Despite having a reset day of 27, all the on-screen month stats seem based on day of month rather than days since data count reset so it's all a little muddled and meaningless anyway. The data usage figure was essentially the same as the when I last saw it on 26 March, so the modem was definitely mistaken, even if the clock change is a coincidence.