Devices will often attempt to use whatever networks are available to them, so make sure that when you land at your destination you disable or turn off roaming on other networks in your phone settings.
For iOS you can find it here:
Settings > Mobile Service > SIMs (select your primary SIM) > toggle data roaming off
And for Android:
Navigate to your phone's settings, then to "Network & internet" (or similar, like "Connections" or "Mobile networks"). Look for the "Data roaming" setting (it might be under "Mobile network settings") and toggle it off.
This will avoid an unexpected roaming bill from your home network.