Installation versus activation
Installation downloads the operator profile to the phone. Activation starts the package validity. Many travel plans activate when they first connect to a supported destination network, but some start during installation.
Always read the activation rule shown on the selected plan. A generic rule for the country is not enough because packages from different operators can behave differently.
The safest routine before departure
If the plan activates on first network connection, install it at home over reliable Wi-Fi, label it ExplSim and keep it off for mobile data until arrival.
Keep the QR code or manual details available on another device. Do not test the plan by forcing a connection if the destination is already covered by a nearby partner network.
After landing
Select ExplSim for mobile data, enable data roaming for that line and leave network selection on Automatic. Keep the normal SIM selected for calls only if receiving them will not create roaming charges.
Allow a few minutes for first registration. Airplane Mode can force a new network search if the phone continues using its previous network state.
If validity starts on installation
Wait until the remaining validity comfortably covers the entire trip. Install shortly before departure while dependable Wi-Fi and another screen are available.
Do not postpone until the airport gate if you will have no assistance, low battery or unreliable Wi-Fi.