UDP and Link Configuration Changes

I have an app that I wrote which sends P2P UDP traffic between nodes on the network. All traffic is routed over the same WiFi segment through my CISCO DPC3941B.

When I send a UDP packet the server receives it, and the modem immediately changes the IPv6 address on the link configuration. This prevents the reply from the UDP server from getting through.

Does anyone have any ideas on how to configure the router to avoid this problem?