套餐的有效期从什么时候开始算，又是怎么计的

It depends on that plan's trigger. If the terms say counting begins when the device attaches at the destination, or when you enable the profile, installing early costs nothing. If they say it begins at purchase, buying far ahead does lose days. Check the plan details first, and if they are not explicit, ask before buying — it is an easy question beforehand and an awkward one after installation.

Not always. Counted as rolling twenty-four-hour periods from activation, yes. Counted as calendar days, the starting date counts as day one in full even if you began in the afternoon or evening, so the usable time is slightly under thirty days. The difference only matters when the trip is exactly as long as the plan. Add one spare day at the outset and it stops mattering.

The two counters are independent: when the allowance is gone the connection stops even with days left. Your options are a top-up if that plan offers one, or a new plan — and both need a connection to arrange, so do it while still on the accommodation's Wi-Fi. In the long run the least troublesome approach is to estimate realistically and add headroom, rather than buying exactly enough and hoping no day runs heavy.

It can, if the layover country is on the plan's covered list and the terms say the count begins on first attachment to a network, because a phone attaches by itself the moment airplane mode goes off — with no intent on your part. The protection is to keep the profile switched off until you are genuinely at the destination, and to stay in airplane mode during the layover if you do not need data there.

Signal bars tell you the phone is attached to a network, not that you are entitled to use data. When validity ends or the allowance is exhausted, the bars stay normal but nothing loads. To distinguish it from a coverage problem, ask whether it stopped abruptly or degraded gradually, and whether the timing matches the expiry you calculated. If it matches, check the plan status before rearranging settings that are probably not the cause.

There is no fixed number, since it depends on the itinerary. The rule that works nearly everywhere is: never let the plan expire on the day you fly home. If the return leaves in the evening or overnight, one spare day is usually enough. If the route has several connections, carries delay risk, or you are travelling alone without support, two is a good investment — what you are buying is not extra data but the absence of a problem to solve on the most tiring day.

It depends on the model: an attach-triggered plan will not start, while an installation-triggered plan starts immediately. Check the plan page.

Remaining data normally ends with the validity and does not carry into a new plan unless the terms say otherwise.