**FIXED**
I did some testing on a spare RUT240 that I have, and that presented the same issues with the non-functioning SIM as it did in the 950.
I downgraded firmware, which automatically clears all running config on the router and requires that you re-run the wizard - even before the wizard was run, the connection came up, which lead me to believe the fix is a factory reset and fresh config.
I then updated firmware to latest version, and connection carried on working. Again, all things suggesting it was a factory reset that was needed.
I did the same approach on the 950 and it worked. Downgraded firmware, which cleared config. Re-ran setup wizard - both SIM 1 and SIM 2 came back online.