It sounds like you are trying to access the HM from the Internet and somehow AT&T is balking.
When you access the HM from your iPad are you accessing it locally through the LAN via WiFi (using its LAN address like 192.168.x.y) or are you accessing it through its cellular data network, over the Internet, and in through the router? It makes a big difference and I would expect, given what you have described, that the iPad is trying to use the cellular data network.
So, I would suggest you work in stages to get the iPad to function.
1. First, how is the HM LAN IP address defined? Did you configure it in the HM web configuration page? Do you get it through DHCP dynamically from your home router? I would recommend that you configure the home router to always give the HM the same IP address based on the HM's MAC address. This is called static IP address DHCP allocation. You would also have to set up the HM to accept an assigned IP address from the router rather than have one hard coded into the HM. Does your router have that capability?
2. What happens when you just type in the HM's LAN IP address (192.168.x.y) to the iPad browser? Still the same result? Can you make the iPad only use WiFi and only operate through the LAN? If you turn off any cellular data access on the iPad, things should clear things up.
3. Once you have the iPad sorted so that it operates successfully on the LAN over WiFi, then you can move back to working on making it run over the Internet. That would require that you define port forwarding rules on the router, subscribe to a free Dynamic DNS (DDNS) service like no-ip.com, and implement their DDNS client on a computer on your LAN.
-- Mache
