I would imagine that task is quite a bit easier than it was in Jolla 1 days when it comes to wider support.
More SoC's with modems supporting wider spectrum of LTE frequencies. Though I guess the US operator situation is still a major headache, if manufacturer like Samsung still needed to do 3 different LTE versions of S5 just for US.

Lets wait and see if we will actually get Jolla 2 first. They still needed crowdfunding, that isn't too reassuring to me. I would think they, and us hope that there would already be 3rd party making hardware.