Call.

It's possible that IAH-NRT is available, but not displaying on the website.

It's also possible that IAH-NRT is not available, but DFW-IAH-NRT is showing a false positive and will not confirm.

If the agent can book you IAH-NRT, do it. If not, try to book DFW-IAH-NRT on your own. If that works, book it, then later have them remove DFW-IAH (there's no fee for changes as Plat).