Faster battery drain is common when your own SIM and a travel eSIM are both on abroad. Each line looks for a network, so the radio does that work twice. The plan is not broken. Set the lines' jobs first, then deal with battery.
Set the default lines once
1Name the two lines differently
When you install the eSIM, give it the trip's name and leave your SIM with its own name. Read the name before every toggle so data roaming is not turned on for your home line.
2Make the eSIM the default for mobile data
Open Settings → Cellular / Mobile Data and set the travel eSIM as the default data line. Your number stays available for calls and texts, still priced by your own carrier.
3Turn data roaming on only for the eSIM
Enable data roaming for the travel eSIM. A data-only plan needs that switch, and the data uses the eSIM. It does not bill your home carrier for data. Leave data roaming off on your own SIM. If there is still no connection, continue with no signal after landing.
When the battery is the problem
- Switch off the line you are not using. If you only need data and do not need a verification text right now, switch your SIM off and turn it on again later. Turn it back on when you need a bank text. See your existing number.
- Change the travel eSIM from 5G to 4G / LTE. The control is usually inside that line's own settings, and the label differs by brand. It is a tradeoff between battery and coverage, not a promise of longer life or a faster connection.
- The system low-power mode still applies. It limits background activity. It does not replace the line settings above.