The Librem 5 is an open network phone, not locked to any particular network.
The modem featured in Librem 5 is Gemalto’s PLS8. Two variants are used, PLS8-E and PLS8-US. Here’s the detailed list of bands supported:
Using frequencycheck.com, you can find your carrier under your country to see which bands are supported by them. However, keep in mind that not all of these bands may be available in your area.
To find out which LTE bands are available from cell towers in your area, you can look at cellmapper.net. Additionally, if you call your carrier they may tell you which LTE bands are available in your area but they also may not divulge this information.
If you require an LTE band that is not supported by either of the modems, then LTE will not be available to you.