Shucks, setting the ST536v6 in bridged mode just broke my MioTV setup. I've forgotten that I've got the MioTV set-top box connected to the LAN port of my router hence its no longer able to dial the modem directly.
The temporary solution is to place a switch in between the modem and the router. So the configuration then becomes modem, router and MioTV set-top box all connected to the LAN ports of the switch.
The long term solution is to upgrade the DI-524 to a DD-WRT capable WiFi router and configure the LAN port where I've got the MioTV set-top box connected as a switched WAN port. Thankfully, this can be easily factored in to my router upgrade project.