I am using a Nexus 5 on a prepaid plan.
I bought US boosters for data, talk, and text - spending over $150. I tried using them in Phoenix last November and did not work. So when I came back to Canada, I checked all the Koodo forums and message boards to see what I could do.
When I was in Houston a few weeks ago, I tried all of these things: restarting my phone with and without my sim card, turning off the automatic selection of carriers and manually selecting all the providers one by one (AT&T, Verizon, Sprint, etc.) but none of that worked.
Luckily I was still able to communicate as the hotels I stayed at has wifi.
I'm going to the US again in two months and if this does not get resolved, then I will either ask for a refund and switch to another carrier.