Playing Destiny with two PS4's through one modem [Question]

Hi! I recently bought my SO a PS4 so we could play Destiny together. Everything works well as long as we are not in the same fireteam, but when we team up the one that is not the host (fireteam leader) gets random disconnects.

First we were able to play three missions no problem. Then we had one disconnection during a mission. Switched to patrol, I got disconnected almost 10 times during two bounties. Back to missions, disconnected two times and then we were able to complete the mission.

Disconnects seemed really random, I couldn't really pinpoint any specific reason for them. We both get around 40/5 mb/s (our internet connection is 100/10 mb/s) so speed is not a problem, and we are both wired through netgear's home network extender.

I'm quessing the problem has something to do with our cable modem, any suggestions? I'd also appreciate feedback from others who are playing with two PS4's through a single internet connection. Is everything working alright for you?

**SOLUTION IF YOU CAME TO THIS POST TRYING TO FIND IT:** I restored my ISP modem/router to factory settings to remove any DMZ's and portforwardings I had done in the past, because I read they may conflict with UPnP. After restoring factory settings, I enabled UPnP and left everything else as it was. No DMZ, no portforwarding etc. Everything worked perfectly after this, we played 4 hours in a row without a single disconnect!