Why roaming must be on for the eSIM line
Travel eSIMs authenticate on local partner networks. The phone treats that as roaming relative to the eSIM profile — so Data Roaming must be enabled for that line, even though you bought a prepaid travel plan.
iPhone steps
Settings → Mobile Service / Cellular → tap your Layer 9 / travel eSIM line → turn Data Roaming ON.
Set Mobile Data to use the travel eSIM while abroad. Keep your primary line for calls/SMS if you want.
Leave Data Roaming OFF on your home SIM to avoid home-carrier roaming charges.
Android steps (wording varies by brand)
Settings → Network & internet → SIMs / Mobile network → select the travel eSIM → turn Data roaming ON.
Choose the travel eSIM as the data SIM. Keep roaming off on your home SIM.
Still offline?
Confirm the eSIM line is enabled, mobile data is assigned to it, airplane mode is off, and you are in a covered country for that plan. Then revisit Data Roaming on the eSIM line.