Two settings, both on the travel line. Turn data roaming on for it — a travel eSIM is always roaming, so this is what lets it use a local network. Then set mobile data to the travel line.
Leave your own line switched on if you want calls and texts on your own number, and turn data roaming off for that line so your carrier is not carrying data.
Give it a minute or two to find a network. If nothing happens, turn flight mode on and off.