Ideally there should be good defaults, coming from this package:
If defaults are not good for you, which seems to be the case, then if you want to make it work for everyone in the future you can contribute changes there, adding updated info about your provider. That way, you help everyone who is using that provider.
The above is upstream so it is best to make the changes there, then there is also the corresponding Librem 5 packaging repo, here: https://source.puri.sm/Librem5/debs/mobile-broadband-provider-info