It could be a number of things. You didn't provide enough details to know, because it is important to understand
if you are still within your radio's network or not. The SIP contact needs to be changed accordingly, plus the ports
need to be forwarded.
I suggest that you drop me a short mail to dj0qn (at) darc.de and I will send you my networking checklist that walks
you through the process of getting this working.
73,
Mitch DJ0QN / K7DX