You have to be in one of T-Mobile's refarmed areas to get 3G/4G speeds, as the Lumia 920 doesn't have frequencies that connect to AWS (AT&T doesn't use that). A refarmed area, basically, is where T-Mobile has modified the tech on their cell towers so that phones without AWS frequencies on-board can jump on their high speed network (meaning: All AT&T phones and most unlocked phones from Europe)
That being said, there's no way to check when your area might be refarmed... You could call, but chances are the rep on the phone wouldn't know either. If the phone consistently stays edge everywhere you go and you can't deal with it, I suggest you wait for the Lumia 925 coming soon to T-Mobile (no known date yet)... Or you can look into getting an unlocked Rogers 920 (Canada) that might have AWS... You're looking for a phone that accesses data on the 1700mHz band (technically, 1700/2100. The 2100 is for uploads)