The EU has certainly brought us one good thing: „roam like at home“. Telecom providers have to let you use at least part of your telephony / SMS / data volume abroad. You can read the exact information about the regulation here. You should find the exact numbers in the app or in the customer portal of your provider.
Using apps like TraceForce abroad has become a lot easier and cheaper, and the worries about data volume and possible costs are gone. For many players in Europe, going to MilSims abroad is a routine and using the app makes sense, especially at such large events.
It is also a wonderful opportunity to be able to choose the network operator abroad yourself. So you can choose the network with the best quality and coverage for the site.
Unfortunately, changing the network used does not seem to work inland.
Bonus tip: If you e.g. have a SIM of your employer, it can also be pleasant to have it with you at all times (e.g. via dual SIM).
On my Ulefone with Android 8.1, the network operator selection can be found here:
Settings → Network & Internet → Mobile network → Network operators
On my iPhone 7+:
Settings → Mobile network → Network selection → Deactivate „Automatic“ and select network.
In the CCL in Hungary e.g. Telenor worked very well, while the other providers didn’t seem like a good choice. To provide an overview and prevent the question „which provider?“, fields can also be supplemented with a list of functioning providers (starting with version 0.8).