Hello and welcome to the community
One of the reasons might be your network providers - please double check with them whether you need to activate roaming in anyway or set the APN settings manually. Please double check your APN settings.
As a (temporary) solution, you might try to get a local SIMcard and check whether this could be working.
Edited to add that this support article might be helpful
https://support.fairphone.com/hc/en-us/articles/9498003764113-Troubleshoot-the-mobile-data-connection