I'm trying to get my server to accept RDP from a different router/gateway than the server is set up on. If I change the gateway on the server to the router forwarding RDP then it connects but if I switch back to the original router, it won't accept the trafic from the RDP router. Any idea on how to get this to work?
I gather you have two routers connecting to different ISPs and you want incomining connections from both to successfully work? You either need a specialized router for this, one that supports dual-wan, or you need to setup NAT on the incoming connections, as the connections hit the RDP server they appear to come from the router instead of some external address. The challenge here is that low-end routers that you would see in a SOHO don't have any facilities in the user interface to accomplish this, most dual-wan routers have this facility built-in.
When your RDP sends reply packets the routing table will be used to choose which router to forward to. NAT on the incoming connections can fix this since each router adjusting the packets so the source address/port is the LAN side ip address of the router. Since the router will be on the local network the RDP server will reply to the correct router.