GTA V - 2 PC's trying to get online at the same time.

The problem:

My girlfriend and I both own copies of GTA V which we purchased from the Rockstar Warehouse. Whenever we both try to get onto GTA Online at the same time, only the person who starts first is able to stay online consistently. When I'm online first and she tries joining a random session, it will either time-out because of "an unknown network error" or she will join a session with very few players (less than 5). When I'm online and she tries joining my session, it will either time out because the same "unknown network error" or she will be able to join for a short amount of time (less than 10 minutes) before it says all other players have left the session and she is now in a session alone. These same issues occurs vice-versa when she is the first one to join.

The setup:

My modem/router is a Netgear N450-100NAS also referred to as CG3000DV2-RG.

My ISP is Brighthouse.

There is a TP-Link TL-SG105 5 port gigabit switch between my desktop and the modem/router.

My girlfriend's laptop connects to the same switch or WiFi, either connection seems to render the same symptoms.

What I have tried:

I started by contacting Rockstar support, they were able to provide me this link which all the way at the bottom tells which ports need to be open. I told them that with my understanding of how ports work, it isn't possible to open the same ports for 2 computers. Rockstar then told me to contact my ISP and closed the ticket. My ISP won't provide free tech support since I purchased the modem from Amazon and it's not leased from them. I have tried forwarding the port to one computer, then making the other computer the DMZ host, this didn't solve the issue. Next I tried enabling Port Triggering for the required ports but that hasn't seemed to make a difference either. I made sure that both games are running the same auto-aim settings and this seemed to help get both of us into the same session but the second person always gets kicked after a few minutes.

What I need to know:

Has anyone had a similar issue, and how did they resolve it? If not, is there any way to open ports for 2 computers on the same network?

Thanks for taking your time to read this and providing feedback. Please let me know if any more information is needed for further troubleshooting.