Hi,
I'm not sure why your firewall config isn't working. I haven't had any problems setting up my firewall using the LuCI web interface. Have you tried this:
config redirect
option src wan
option src_dport 3389
option proto tcpudp
option dest lan
option dest_ip 192.168.1.2
This leaves out target 'DNAT' and dest_port '3389'.
If you're new to firewalling, port forwarding and Network Address Translation, please take the time to read more on the Internet. You may be able to solve your own problem!
Good luck
Clemmitt