Cannot join same server with wife under same internet

I am having the same issue with my brother. Just wrote, "Bought myself and my brother who both live in the same house copies of BF6 through Steam. When we try to party together and join a match at the same time, with friends too, one of us is always left in the menus while the other joins the map. He uses WiFi. I use Ethernet. When he uses a VPN, we are able to play together, but I experience a ton of lag. UPnP is enabled."