Unable to connect LG G3 T-Mobile phone to networks in Europe

I recently brought my LG G3 with me to Europe (while on Honeymoon) because I found out from T-Mobile that I could use text and data there at no extra cost. My phone works fine in the United States. But it doesn't find a carrier connection in London; Southampton; Vigo, Spain; Lisbon, Portugal; Cadiz, Spain; Gibraltar; and Barcelona.

I tried my phone in each location and it never connected to anything that wasn't WiFi. That really really sucked. I get that I have it rooted and I am using a different Lollipop ROM than stock. But I didn't think that would prevent it from connecting to towers there.

So what happened? What would be on a fully stock T-Mobile LG G3 that I wouldn't get on my rooted and ROMed version? I'm sure it something simple, but I would really like to know so that I can fix the problem before I go back.

Unlocked US model in Europe

Have been searching the whole day, but is there any difference in supported LTE bands between the US and Europe model? I haven't found any differences... Seems there is only one unlocked model? (Differences aside from RAM).

Visiting New York in January, this looks like the ideal phone for my girlfriend… Hope to bring one home for her.