Hello,
the TUMPA has Configurable output of "RS232 (MAX3243) level" or "TTL level" for serial communication
Please see:
http://www.tiaowiki.com/w/TIAO_USB_Multi_Protocol_Adapter_User's_Manual#Serial_RS232.2FTTL_level_Selection_Jumper
for 3V-3.3V routers, you need to use TTL level, and for 5V routers you need to use RS232 level.
According to DD-WRT Community forums your router has 3.3V level, so you need to make sure TUMPA is set to "TTL-OUT" and then use "TTL Level Serial Connector" on TUMPA for connection to router.
If you dont trust voltage level information from DD-WRT community, then you can test voltage on serial pins using voltmeter/multimeter to be sure.