The ONLY way to fix this is to upgrade your T-Mo Internet plan to the 19.99 / Month plan. The 5.99 / month plan is a proxy server that is only open on ports 80 (web) and 20-something (e-mail). YouTube is run on a different port, and is not forwarded in the 5.99 plan. (Same with some of the IM solutions on Installer- they use different ports not supported in the 5.99 plan).
Should work fine on WiFi.
So you need to ask yourself- is YouTube worth 14 clams a month for the upgrade.