Orphee There is an open issue here: https://github.com/GrapheneOS/os-issue-tracker/issues/956. I believe that one of the overrides that are mentioned would include enabling 5G, but I could be mistaken.
The issue is still open and is marked as an enhancement with no priority. Overriding carrier configs might be added in the future, but the developers usually find themselves busy with other higher priority items, like supporting new devices, preparing for big QPR or major version releases, or other important privacy or security improvements.
So, for this particular feature, one that gets some special attention sometimes, I'd once again remind people (not to anyone in particular) and anyone who might be interested in this feature that GrapheneOS is an open source project. Someone who wants the feature and can put in the time and effort to make a good implementation that meets the project's standards is welcome to take a crack at it.