As usual, as much information as possible. Full config export can’t hurt, although if it’s too long, it’s easy to miss something. And to be honest, it’s no fun going through hundereds of lines of text. It’s different when it’s on router, because there are live counters and such. The fact is, you’re in much better position to find anything than anyone here on forum.
Try to find out what’s going on. Does the marking rule get any hits? Do you see packets going the right way? It’s very easy to test it on live system. Try to connect to some unique address or port (it can be made up, it’s just for easy filtering) and use Torch to see where packets go. Or add logging rules to prerouting/postrouting/forward to see where the packet may get lost. If you see the packet going the right way, make sure that addresses are correct, e.g. you might need to use srcnat, because remote server might not have route to your LAN. Try stuff like that, it’s not too hard.