I had the same on US Mobile’s Dark Star (AT&T). It was working then all the sudden it stopped. All the APN settings looked fine and matched the website’s recommended APN setting (as well as our forum discussion on the APN Setting), but it just would not access mobile data. In the end, I got it working again.
In the APN Settings on BraX3, I reset the APN to its original settings (3 dots top left) and created a new APN from the Carrier’s website. I don’t know why this worked, but it did. I checked the APN it was exactly as on the website before I reset it and up until recently that very APN worked fine. Anyway, it works now and is on 5G. Hope that helps your situation.