I have a strong suspicion: you need to activate the data connection on each network and then configure the necessary settings.
Something like this:
- Activate data connection on SIM 1
- Select primary data connection
- Set roaming for both SIM cards (enable/disable)
- Repeat the process with SIM 2
- Repeat the process steps in the roaming country with both SIMs
I have installed Iode for the second time and each time the problem has improved over time. My assumption is therefore as above.