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APN are the names applicable to get access to the mobile network of the carrier one subscribes to. Threads on the subject have the hashtag apn ← is a link.
Post #4 offered a suggestion. Ofc you are paying for all the services on your carrier’s network so one would first turn to the carrier’s support systems to enquire about the availability of their services.
Significantly your carrier will publish their APN usually with a guide on how to apply them. As roaming can be expensive it it a good idea to check the T&C of your plan before travelling. You should not really be expecting to change APN as you cross a border, the carrier is expected to arrange service in the new country.