Use search: there are at least 3 ways to make HTC TyTN (any any other WM6 phone) working as Internet source for ITTs:
1. The best, but a bit complex: configure BT PAN on n810 to use Internet Sharing app via bluetooth. I wrote a draft guide what to do. I'm using this method.
2. Install BT DUN profile support on TyTN - works, but as it's not officially supported has some side effects, like no way to control the connection on WM, no way to count data, you cannot use internet on TyTN at the same time.
3. Use WiFiRouter application (not free any more), that acts like a WiFi router, that you connect to via WiFi and share Internet connection this way - the biggest drawback is huge power drain