Well, I did something and it's been running all night!
I put another £10 on the SIM card! It should not have been needed but perhaps Voda are expiring their PAYG SIMs (I originally put about £50 on this one; it was supposed to be good for 20GB and I used maybe 1/1000000 of that) after much less than the usual 90 days.
But how can this possibly work at all?
About 20 years ago, when contracts were expensive and I had Voda PAYG SIMs in various devices I was travelling with, I discovered that Vodafone allow very small packets through on expired SIMs. Very small - probably under 100 bytes. But not always; it works perhaps once an hour.
So a DNS lookup tends to work, but longer stuff, like doing a WGET from an HTTPS website (TLS involves some kbytes of data for anything), no way.
Smallest possible topup appears to be £10.
For any real commercial use, people will use a contract SIM, of course. You can get them down to £3/month. PAYG still makes sense in many very low usage scenarios but the networks try to block that usage, so things like alarms have a feature to send an SMS or make a call every say 89 days, and then a £50 or so topup should work for ever. So they block that by requiring a topup every 90 days, etc... Currently, with Vodafone, an SMS every x days does seem to work for years (I had one such system running). Mobile data usage does not extend the SIM activity.
I also configured the RUT240 to reboot every day at 2300, as recommended by lots of people. It's a good procedure, because it deals with bugs like memory leaks. It seems to be a linux box, with an extremely slow file system (any config save takes tens of seconds) so perhaps implemented in the CPU FLASH. A lot of chinese boxes are like that... very very slow config page loading, saving, etc.