Next step would be to master reset the device and retest. Be sure to back everything important up first.
Be sure to test the wifi first thing before any customization or application installation.
I've got the same issue. I did some searching and found a Microsoft forum where this is also being discussed. You may wish to weight in there, too. However, it is mostly up to T-Mobile to raise this issue with its vendors (HTC and Microsoft).
In case T-Mobile support is reading this, here is the information for my phone:
I end up having to turn WiFi completely off, otherwise data access doesn't work or is extremely slow. Sometimes turning WiFi off and back on cures the problem, but only for a brief while before it stops working again. I've tried multiple routers now, but I've listed my primary one below. Interestingly, the "Internet Sharing" feature on the phone works great; it's the reverse of this -- using WiFi access points to connect to the Internet rather than going over the cellular connection -- that isn't working most of the time.
Wireless router: NETGEAR Router WNDR3700v2
Wireless router firmware: V188.8.131.52
Wireless configuration: WPA2, broadcasted SSID, 802.11a/b/g/n
Phone model: HTC Radar 4G (T-Mobile US)
Mobile Operator: T-Mobile US
Mobile OS version: 7.10.8107.79
Firmware revision : 1600.2200.20402.531
Hardware revision: 0002
Radio software: 16.23.05.13_2_16.30.00.23U
Radio hardware: 2.16.0.D4
Bootloader : 2.4.160015.3(136036)
Chip SOC: 184.108.40.206
PC OS version: Win7 x64
Zune version: 4.8.2345.0
Country: United States