I don’t work for Purism, so I can’t confirm for sure precisely which frequencies it supports, but the campaign page makes the following claim:
WHAT COUNTRIES AND MOBILE NETWORKS WILL SUPPORT THE LIBREM 5 PHONE?
The Librem 5 is an open network phone, not locked to any particular network. It will work in all countries that provide 2G/3G/4G, GSM, UMTS, or LTE-based network services.
Personally, I made the assumption that this meant it will support all possible bands, because not all networks use all bands. It would be nice to see confirmation of the supported bands.