iPhone connectivity issues abroad

You are right. There are about three different models of the iPhone X, each supporting the cellular networks of different regions of the world.

According to GSM Arena https://www.gsmarena.com/apple_iphone_x-8858.php, they are:

• A1865 (USA, Hong Kong, Australia, New Zealand, China)
• A1901 (EMEA, UAE, LATAM, Canada, USA - AT&T/T-Mobile, Singapore)
• A1902 (Japan), A1903 (Unknown market)

You can see that all of Europe uses Model A1901, which is probably the one you own. If you get a T-Mobile SIM card in the U.S., it should work with that model, as well.

To check which iPhone model you have, you can navigate to:

Settings > General > About and tap on the Model Number line to make the Model number appear.

It's the same with the newest iPhone 11 (Pro). You can check the regions they support under:

https://www.apple.com/iphone/LTE/