I posted above about how data might not be carried from one EU country to the next. Since then I came upon this at thesavvybackpacker.com and the author knows more about phones in Europe than I do.
As of 2017, the EU passed a “roam like home” policy that bans mobile carriers from charging mobile roaming fees for any EU member country. In short, any eSIM data plan bought in the EU can be used in any other EU country without extra fees or restrictions. This is great for travelers because you can use one plan everywhere.
Here are the countries/areas where there is no data roaming: Andorra, Azores, Aland islands, Austria, Balearic Islands, Belgium, Canary Islands, Cyprus, Corfu, Crete, the Cyclades, Croatia, Czech Republic, Denmark, Estonia, Faeroe Islands, Finland, France mainland, Germany, Gibraltar, Greece, Guernsey, Hungary, Ireland, Iceland, Italy, Jersey, Latvia, Liechtenstein, Lithuania, Luxembourg. Madeira, Malta. Isle of Man, Norway, Netherlands, Poland, Portugal, Rhodes, Romania, Spain, United Kingdom, San Marino, Sardinia, Sicily, Slovakia, Sweden, Vatican