There has been some discussion about how to help people with reluctant carriers and one idea has been to modify the Settings app to incorporate override switches for VoLTE and 5G. It certainly is not top priority amongs the GrapheneOS developers. I'm not savvy with Java, so if you, the reader, could consider helping us out here, it would be great :)
I'm the one who is always bugging people in the Matrix channels about my shitty carrier that has not updated carrier configs in ages.. Or it's because Google does not officially support Pixels in Finland for example, so it never requested adequate configs through official channels.
Related GitHub issue is here: https://github.com/GrapheneOS/os-issue-tracker/issues/956