I would edit this into my existing comment, but alas one cannot even see comments that are still in the pre-moderation queue, let alone edit them. :(
After some further digging, I managed to track down the user manual for the Npower specific variant of the Geo Duet II. The Npower website may be long gone,
but I'm not defeated that easily.
Apparently though, you're not the only people who've noticed the Duet II randomly stop working after switching supplier. After digging through the archives of my home forum, I came across some notes that were left a while ago. If you're seeing "Connecting to Smart Meters" it can apparently take anywhere between five minutes and one hour for it to connect and start showing data - especially for gas meters. Rebooting the device will slow this down as the timer effectively resets.
I'm also seeing indications from other forums of similar issues as well, where someone was originally with Npower and later left but were able to use the Duet II just fine with multiple suppliers in the time since until a few months ago when it randomly "died". Soooo... It could just be wear and tear? It seems to happen a bit more often on the Npower variant but I don't have enough data to know for sure - it could just be that people with that variant are more vocal than people with the vanilla one for example.
It also seems that these days, Geo only offers somewhat limited support with regards to the Geo Duet II because it's such an old model and has been superseded repeatedly over the years since. Things have changed a lot since 2013 after all. I guess you could try to have it unpaired from your meters and paired up again if you speak to customer services, but I can't even say for sure whether they'd be able to get it to pair up again, so this is something you'll only want to do if you're prepared to take that risk.
If that still doesn't work out, them I'm afraid I honestly can't think of any other solutions as I'm already scraping the bottom of the barrel here. You may just have to accept that the Geo Duet II is little more than a decorative item at this point.