Setting MCC/MNC in Lumia 1520

[Windows Central Question]

My carrier's (T-Mobile) tech support suggested I would get better data performance if MCC/MNC were configured, but it's not displayed in Windows 10 settings. The Field Test menu's GSM monitor displays them set to 000-00. Surely there's some way to access and configure those fields. Is there some other short code that actually works on an unlocked AT&T phone?