VPN Through Bridged Network Coections to AT&T Wireless Personal Hotspot on iPhone

The DSL coection my office usually uses is currently offline due to an service outage and I'm trying to tether to a phone as an emergency fix.

Here's what I'm doing:

I've got the Inteet coection propagating across the network, but I caot establish the VPN coection. It seems that either (1) the AT&T wireless network or else (2) the bridging PC is preventing the VPN traffic from getting through.

AT&T support denies they're blocking any traffic.

Is there a setting in Windows on the bridging PC that I should check?