Most connectivity problems abroad are not caused by the data plan. They are caused by things that should have been done at home. This list takes about ten minutes.
Before you buy a plan
1. Check the phone supports eSIM. Dial *#06#. If an EID line appears, it does. If not, an eSIM is not an option and you need another plan.
2. Check the phone is not carrier-locked. Phones bought on a carrier contract abroad are often locked and will refuse another provider's eSIM. Ask the operator that sold the phone — there is no more reliable check.
After buying, before flying
3. Install the eSIM on home wifi. Installation needs an internet connection. A foreign airport after landing is the worst place to discover a problem.
4. Do not switch it on yet. Install it and leave it. Only enable it on arrival — for most plans the validity period starts when the eSIM first connects to a network.
5. Download offline maps. Google Maps lets you save a region to the phone. Do it for the cities you are visiting. It rescues you in every loss-of-signal situation.
6. Screenshot the important things. Hotel address (in the local script too), booking reference, return ticket. Screenshots work with no connection.
7. Install the apps you will need. Ride apps, transit apps, local maps. Install and sign in at home. Some countries use apps you have never heard of.
About your home number
8. Turn off mobile data for your home SIM. This is the single most important step for avoiding roaming charges. Settings → Cellular → your home SIM → turn off cellular data. You still receive calls and messages.
9. Check how your bank sends verification codes. By SMS, you will still get them (your operator may charge a small fee abroad). Through an app, it needs data — which will run over the eSIM, so it works.
10. Tell your bank you are travelling. Nothing to do with connectivity, but it belongs on the same list: cards blocked for unusual foreign activity is a common way for a trip to go wrong.
Once you have landed
Switch the eSIM on. Turn off airplane mode. Give the phone about a minute to find a network. If there is still no signal after two minutes, restart the phone — that fixes most cases.