I've got two routers with two separate public IP addresses on the same subnet, but I can't get them to talk to each other. Both are connected to the internet (ISP-provided gateway) via Ethernet ports provided by the landlord, but I don't have access to or knowledge of how those are physically connected or the protocols used to get back to the ISP. I can ping either from the outside, but they can't ping each other. Traceroutes in and out look the same, and they receive the same gateway over DHCP. I can ping other IPs on the subnet, so I assume this is not any sort of intentional isolation for security/privacy.

Since I'm in a setup where my landlord provides internet and we don't have contact with the ISP, I can't really ask the ISP for help (doubt the landlord would know much either.)

The situation is similar to the diagram at this question, but instead of the two servers, there's another router coming off the (presumed) switch, and I don't have access to the switch.

I've tried giving them static routes to each other with the ISP internet gateway as the gateway, but that's not working. One is a Linksys WRT54GL running DD-WRT, the other is a Netgear WGR614v7, although I could get something more capable if necessary. I'd like to keep them each connected directly to the ISP on their WAN ports, but I can have an ethernet cable between them if necessary - I'm wondering if there's a way without that, and if there isn't, I'd appreciate advice on how to get that working.

Sorry this is so nitpicky; there are reasons for all the constraints, but they don't apply to the real question, so I left them out. ;) Thank you!