Finally found the way to use static routing.
This works for me (DIR-655, fw 1.11):
-Go to Advanced/routing menu
-Paste this to address line of your browser (everything is one line!):
http://192.168.30.1/Advanced/Routing.shtml
?config.route_table[0].enabled=true&config.route_table[0].used=1&config.route_table[0].entry_name=RA25&config.route_table[0].dest_ip=192.168.31.0&config.route_table[0].metric=1&config.route_table[0].dlink=2&config.route_table[0].subnet=255.255.255.0&config.route_table[0].gw=192.168.30.30
-Edit values to meet your needs (entry name, dest_ip, subnet, gw)
-Hit enter and voila, IT WORKS! (I've done this using Firefox/Iceweasel, not tested with other browsers)
Table still shows "WAN" as the interface, dont get confused. Never attempt to change settings through that page!
URL Above will change the first record in the routing table. For subsequent record use different index at each field ( [1] instead of