From a site selling them unlocked it looks like standard international bands. Unfortunately no T-Mobile USA.
HUAWEI E5331 unlocked 21.6Mbps MiFi mobile broadband hotspot router specifications:
HSPA+ DL up to 21.6Mbps
HSUPA UL up to 5.76Mbps
HSPA+/UMTS 2100MHz
GSM/GPRS/EDGE 850/900/1800/1900MHz
MicroSD Card slot(32G extension)
1500mAh E5331 battery
Wi-Fi 802.1b/g/n
one simple touch to surf the web
Up to 8 devices access simultaneous
Dimensions: 92 x 60 x 12.8mm
OS: Windows XP/Vista/7,Linux, Mac OS.
E5331 Router hotspot is another stand out unit from E5 series as one of the 21.6Mbps devices, it is supports up to 8 users connection at one time, besides its 12.8mm ultra-slim delicate design also the highlight between others.