ATT Passport plan is $120 and it does work, but data is small. What tourists do is to wait for hotels and local tour buses with wifi. At each cruise ports, 100s of passengers and ship staff just sat in dock reception halls to use wifi. I have friends who stay on hop on hop off buses without looking at anything but their phones.

You time is short in London and Paris, maybe not worth it to buy local SIM cards. Wait until Italy, find a local phone shop to buy an Italian SIM card with data.

Tmobile used to sell service without contract, $50 unlimited global internet, you can cancel after you come home. But it stopped that 3 months ago, you can still go into your local Tmobile to see maybe still available in your city.

If your first stop is the uk - you could buy a "three" network payg sim - France and italy are charged the same as the uk (with an add on - iirc £15 for a month) for calls text and data.

Monaco isn't included though...

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