how to fix Conversation IP violation

At our school the VDSL doesn't go so well, so we joined 2 VDSL to get enough bandwidth. Joining two Internet connection means that several sockets inside a "session" are generated by different IP addresses. In our case two different ones (but static). This makes us impossibile to use Unity

To overcome issues similar to this we have already in place a system to use only one connection for packets going to a domain, eg. unity.com.

But it didn't work.

Then we tried limiting to one connection, all the computer that needs to access Unity. It worked, but is not acceptable.

Probably some checkup needed by unity is outside the domain unity.com Does anyone know how to fix this?