Rookie (aka QPR 2.0 beta)
Upstream QPR2 has been released and the developers are working on the GrapheneOS update now.
I don't know how this works exactly, but I'm assuming that the update you're looking for is in the carrier configs, not the OS. I believe these are separate things, or at least they do appear to be in GrapheneOS release notes, like in this one: https://grapheneos.org/releases#2024120900
adevtool: update carrier_settings and carrier_list protobuf parsers for Android 15 QPR1
I don't know if the next release will have the same kind of update, but based on the "Android 15 QPR1" part of the quoted line above, I think it's likely we will have a carrier settings update soon. Assuming support was added as you say it was, I would expect it'll start working for you when the update comes through.
Note that the change I pointed out above wasn't part of the first update after QPR1 was released. I don't see why it would be any different with the upcoming QPR2 release.
That said, I want to say again that I don't know how this works exactly, but hopefully I've understood all this correctly and it'll be working soon.