I solved it after 2 days of fiddling
The route table had 2 default routes
nevj@trinity:~$ ip route default via 10.0.0.138 dev eno1 proto dhcp metric 100 default via 192.168.44.1 dev bnep0 proto dhcp metric 750 .............
I had to delete the first one which was the nonfunctioning modem connection. So
ip route del default via 10.0.0.138
then it would use the second, which was the mobile phone link.
NetworkManager made the 2 default routes, and that should not be a problem. If the default route with the smaller metric is down, it should use the second default. It did no such thing.
Now maybe I can get something done. I am writing this via the mobile phone link.
Cheers
Neville