In order to avoid any unnecessary roaming charges from your primary carrier, you will need to make sure that your primary SIM line is not active. In order to do so please make sure your eSIM line is active, while simultaneously making sure your primary SIM line has been deactivated manually. If your primary SIM line has been deactivated, you will be unable to use data on that line (and consequently will not be charged any additional roaming fees from your primary carrier).