[XBOX360] Can't connect to EA servers with two Xboxs

I'm having a problem connecting two Xbox360s at the same time.

I have two Xbox's, two Gold accounts, and two physical copies of BF4. We've been playing BF4 at the same time for about a year with no issue until last night. Our router died, so we bought a new one (Lynxsys AC1200).

The first person who loads BF4 gets in fine and can play with no issue, the second person cannot connect to EA servers. Googling the problem and looking through old posts here it seemed to be an IP conflict, so we followed the recommended IP auto-assign with UPNP, port forwarding, manual IP assignment, turning off NAT and firewalls all together... everything we could find.

If we load in any other game we can play at the same time with no issue, it's only with the EA servers that we run into a problem.

Obviously I think it's an issue with the router because that's the new variable, but I don't understand how a newer (supposedly better) piece of equipment won't work.