Lorna’s trick did work for me, but I need to reboot the phone after the first step in order for it to work. So my steps are:
- Deactivate SIM1 I don’t want to be 4G (which has options 4G/3G only)
- Reboot phone
- SIM1 now has “2G” option! Set it to 2G and set SIM2 to 4G.
- Activate SIM1.
- Going back, can set SIM2 to 2G and SIM1 to 4G, but again it will only have 4G/3G options, so have to repeat next time I want SIM2 to be 4G.