You are expecting we are familiar with exact behavior of various kind of routers and we can guess it's configuration. Sorry, we can't.
Unless you know behavior of devices you are using, you need to analyze what's happening at network level. It require someone familiar with networking basic as well as basic knowledge of the SIP protocol. Ask your network administrator for help if you have skills required.
As a blind shot - either WiFi router uses SIP unfriendly NAT or the stack of the devices you chained creates it together.