Data roaming first. On an iPhone: Settings → Cellular → the travel line → Data Roaming on. On Android it sits under the SIM’s own settings. Without it a travel eSIM cannot use any network.
Then check that mobile data is set to the travel line rather than your home line, and that the travel line is switched on.
Then turn flight mode on and off, which makes the phone search again. Wait up to two minutes.
If all three are right and there is still nothing, pick a network by hand. If that fails too, write to us while you are still travelling.