OK, so I have identified the fault as phone-related, not SIM-related.
As suggested, I swapped SIMs with another phone that could connect to BTWiFi-with-FON on my Home Hub (with its own SIM).
After the swap -
a) the other phone + my SIM connected to the FON service
b) my phone + the other SIM connects/disconnects from the FON service, just as with my SIM fitted.
Conclusion:- my Samsung A3(2016) will not connect to the FON service (at least on my Home Hub).
Many thanks to all you people out there for your input.
So a final post on this issue.
I booted the phone in 'safe mode'
I used settings/connections/WiFi to find BTWifi with FON - AND IT CONNECTED! Therefore the problem was with an app on the phone.
I deleted several apps until I was able to connect to BTWifi with FON, then reloaded the apps I wanted to keep.
Funny thing, though, the last 2apps I deleted that then let me find a FON connection I have reloaded and there is no problem.
Thaank you again for all your help.