In Europe if you buy a simcard in an EU-country you can use it in other EU-countries. Maybe some blogs will have answers for you what is the best option. https://www.theworldwasherefirst.com/best-international-sim-card-for-europe/
Inaturalist needs
a data connection to recognise species names. Other apps have build in species-lists and can be used offline. It would be nice if this was added to the iNaturalist app too. See this feature request: https://forum.inaturalist.org/t/android-app-offline-species-list-to-add-ids-offline/2179/3