Don't know much about the matter, I am from EU and traveled to NY a week ago.
The carrier I was assigned to was AT&T from my own carrier and it showed like I had 3-4 bars 4g all the time, although everything was so slow, in many cases I couldn't even do a search on google.
For a whole week, while being there I felt like I had no actual 4g, heck not even 3g. I don't know if thats AT&T specifically or New York being a very busy city but ye thats a real problem right there tbh, seeking out wifi just because you cant get the service you are supposedly paying for sucks.
It's mostly because of lack of local break out https://www.dialogic.com/glossary/local-breakout-lbo
Basically your traffic travelling back and forth between roaming network and home network. Next time check your IP and you will see EU IP instead of US. Although some operators offers local breakout, it's still rare.
Are you sure your phone has the right bands? I assume NTT DoCoMo is very fast in Tokyo, but my iPhone (even though it seems to connect at 4G) is agonizingly slow. Like sub- 1 mbps.
Sometimes roaming is throttled (because carriers charge each other crazy rates) so you may not have been getting the throughput that the AT&T network is capable of.
I was on vacation on Spain with my girlfriend's family, who are from the US. They all had booked international roaming from T-Mobile US. Their speeds were so bad they couldn't even look up restaurants on Yelp.
Meanwhile, I am on T-Mobile Germany, which since last year has free (and unthrottled, I think) roaming in the entire EU per EU law. My data worked flawlessly. It was even faster than the hotel WiFi, and I used it to tether to my laptop instead of using the WiFi.
The carrier I was assigned to was AT&T from my own carrier and it showed like I had 3-4 bars 4g all the time, although everything was so slow, in many cases I couldn't even do a search on google.
For a whole week, while being there I felt like I had no actual 4g, heck not even 3g. I don't know if thats AT&T specifically or New York being a very busy city but ye thats a real problem right there tbh, seeking out wifi just because you cant get the service you are supposedly paying for sucks.