The most common miss is waiting until you land to scan the QR code. Airport Wi-Fi is crowded, and some phones want a stable connection to finish installing a profile.
The night before
Buy the plan a day early. Scan the QR on a stable network, label the line with the country name, and leave it switched off until you arrive.
When you land
Turn on the travel line, set it as your mobile data line, and enable roaming for that line only. Your home SIM can stay on for texts.