All steps

1. 1On the Home Screen, tap "Settings".
2. 2Tap "Cellular".
3. 3Under "SIMs", tap your travel SIM.
4. 4Tap "Cellular Data Network".
5. 5In the "Cellular Data" section at the top, enter the APN.
6. 6Tap Back at the top left. iPhone saves automatically.

Phone screens are illustrations. Menu names and positions may vary by OS version and carrier. APN values are examples — use the values from your carrier.