It’s not possible to say with any degree of certainty what happened but there are circumstances that could explain some of the things you’ve experienced.
If as you say the Internet was working, then Mercury would have been able see their side of things working but just not coming out at the other end. Like us, they don’t own the Internet so wherever the problem is, when this sort of thing happens, they can’t actually do much about it. So you’re told it’s an Internet failure because it saves having to go through all sorts of detailed explanations that you probably wouldn’t understand anyway (rather like a doctor telling you you’ve just got a virus).
The fact that they didn’t have to come onto your system isn’t that unusual either. The probability is that if you were down then so were many others and they would have know this. I’m sure that Mercury, like us, can see the Internet status of all their customers and from this information, it’s relatively easy to work out what part of the Internet (if any) had failed.
If it was an Internet failure then there’s a strong possibly that the problem wasn’t even in the UK and may explain why we saw no problems during this period. I suspect though that you’re using roaming sims so your data is being routed overseas making it far more vulnerable to failures in places like Germany or France. All that said, I’d put money on this being some kind of failure on the computer that allocates the connections and this would explain how some cars were seen to be working while others failed to connect. Back in the UK, your mobile device was probably seeing a cracking signal so would never want to roam to a different mobile provider and that’s exactly why we won’t use them. Anyway that's all horses for courses.
Hope your problems have gone away now and that things settle down for the New Year period.
Bill