Smart SIM will only kick in automatically if you *completely* lose signal on your main SIM.
Otherwise, we recommend switching manually so you can connect to EE, O2, Three and Vodafone whenever you need to.
Here's how:
1. Swipe down from the top right of your screen to open Control Centre
2. Go to Mobile Data or Cellular Data
3. Switch your data line to Smart SIM
Follow the same steps to switch data back to your main SIM when you're ready.