Not sure about the WR21 - but on the WR31's I use a DB9 slim line null modem/M-M adapter. This allows direct connection of regular serial cables with no issues. Otherwise it is a chore trying to figure out the pins, cable type, etc. Search online for
AD-N05M-2. Of course, you still have to setup your serial parameters. Not sure what your trying to use off the serial, but perhaps this may help.