Hello Emilekamdem,
Welcome to the community!
Are you able to use your phone for other services? Are you able to use any data at all (ie '3G')?
Have you verified that your phone's data roaming setting is enabled? You can view those settings: Settings --> Cellular --> Cellular Data --> Data Roaming.
Have you tried to manually select a network on which to roam? You can view the available networks: Settings --> Carrier. It's possible some of the networks do not use the same LTE bands/frequencies available on your phone. Manually selecting a different provider might allow your phone to connect to the LTE network.
Have a great trip!
Hope this helps 
Cheers
