I just tried my $30/mo 100min/unlimited text/5GB plan in my iPad and just like the above poster could only get to T-Mobile's website.
Every APN other than fast.t-mobile.com I tried with the iPhone SIM card wouldn't get me anywhere, not even to T-Mobile's website.
Luckily I had an unactivated T-Mobile SIM lying around and used it for a 200MB/mo free plan.