If User is an AT&T user verify under Cellular settings that both the primary and “travel lines” have data roaming enabled. Seems as if the default setting for the travel line is to not have roaming enabled, so you will have to manually enable it. After enabling data roaming switch phone to airplane mode on and off so that it checks back in with the local cellular network then test. Ensure the phone is not connected to Wi-Fi as It should now be connecting to the network successfully and receiving data. A good test is social media platforms such as Instagram or Facebook as the web browser may have a bit of a spotty connection due to the way that it handles data.
This may work with Verizon and T-Mobile as well but has not been confirmed.