Layovers and transit: staying online without wasting a plan

Technically yes, but if the plan does not cover the transit country you burn validity for nothing. Start it on arrival at the real destination.

Compare a regional plan covering every country on the route against the sum of single-country plans before deciding.

There is no fixed number, because the shape matters more than the length. Eight hours airside may need nothing at all, while three hours that involve reclaiming a bag and checking in again on a separate ticket can justify a route online that does not depend on Wi-Fi. Ask whether you will leave the terminal and whether anyone needs to reach you before asking how long it is.

Only if the transit country appears in that plan's country list. If it is not listed, the phone will not connect there, and activating at that point can also burn validity for nothing. Check the country list before you travel.

It depends on whether you need calls or SMS on that number. A common arrangement is to leave the home line able to receive calls and SMS while turning its data off, so the phone cannot use roaming data unintentionally. What roaming costs and on what terms is set by that operator, so check with them before you travel.

It is a public network, so assume other people are on it. General reading is usually unremarkable, but for signing in to important accounts or moving money, your own mobile data or a network you control is the better choice. Be wary too of networks named to look like the airport's official one.

Write out every country on the route, then compare a single regional plan that covers all of them against the sum of country plans for only the ones where you genuinely need data. It often turns out that some countries on the route need nothing, because you never leave the terminal and are not there long.

First, get the airline's own answer on what it will do, because that varies by carrier and by the cause of the delay. Second, if you will end up leaving the airport you need real data to find a bed and get there, and in that situation a short plan for that country usually beats hoping for Wi-Fi you cannot locate. Keep receipts for everything in case you can claim through the airline or a travel policy you hold.