iPhone connectivity issues abroad I have a fully functional iPhone X which works as expected in Sweden. However, when traveling to some countries I experience connectivity issues. 

Eg. in the US I get no reception at all, whereas in Ireland I get 4G connection which works well for data, but can't seem to place voice calls most of the time - when it works occasionally, the call gets dropped after about 30 sec following weird garbled sounds. When I visited France on the other hand, I experienced no issues at all.

I might add that this is not a problem with my service provider as I have tried several SIM cards (even bought local ones) which all exhibit the same issue.

This leads me to believe that it might be an issue specific to band frequencies, but I understand too little about the subject to troubleshoot the issue. Has anyone had similar experience and can point me in the right direction or the right (focused on mobile tech or similar) forum to ask?
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&amp;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/