Progress of a kind. I tried the dongle again. The AntTester app reports that the Ant Radio Service needs the use Ant hardware permission. That doesn’t happen on my S2 where, as above, everything works fine. However, the actual permission is not visible on either tablet.
This looks increasingly like a Samsung/Android issue. There are similar reports of problems with Samsung tablets on the TrainerRoad forums. TrainerRoad support have responded with a fix however - not sure how they managed that. I only use Zwift so it’s hard to figure out more.
I’m currently looking into an Ant+/BLE bridge. There’s also a bridging app, but it needs a mobile device that does handle Ant+ (possibly via OTG), and none of mine now do. Also that app is not in the Play Store, so has to be installed directly.