ok - I admit to not knowing a lot about 2g, 3g. ... but i did try a quick google and found that amazon claim to offer close to 100% 3g support for paperwhite in UK. it was specific coverage info for 3g paperwhite, not a general 3g thing. you go via the paperwhite product details web page which has a link that takes you here
https://client0.cellmaps.com/viewer....iew=intl&cov=3
only other thought iso check if there there is some setting you accidentally nudged that tells it to go 2g if the signal is stronger, or some such ?