I had the same problem. The only way i have found was:
Create a new WebRequest and request e.g. this page: http://checkip.dyndns.org
then parse out your ip address from the reply.
This gives you your official internet adress if you are
behind a NAT-router at home.
There are more of this "my ip"-pages. Just google "my ip".
It is also recommended that you have multiple pages if one
failes. When you get a timeout at dyndns , request another
my-ip page.
Or if your router has a admin-webpage, request the
status-page and parse your ip from that page.