Same Internet/IP cannot play together

My internet connection has no issue. I have 2 PCs trying to play BF6 together on the same modem but one of us will either time out or experience tremendous packet loss. Both PCs are connected via ethernet to the modem. We have no issue connecting to anything else and we can both play solo.

We tried using a VPN on one PC but that only made the packet loss issue worse.

We tried having someone else host, no change.

We tried using a router connected to the modem and only connecting 1 PC to the router, no change.

We cannot play the game together.