js28f640j3 is unsupported by zJTAG. Because zJTAG's upstreams: TJTAG and BrJTAG are not maintained anymore...
OpenOCD is the way to go. TIAO corp, sent test units of TUMPA-Lite and TUMPA v1/v2 to OpenOCD developers and then they added support for TUMPAs and also created an easy-to-use script for flashing modems and routers.
You can contact Paul Fertser, he is friendly openOCD developer & active user and he has TUMPA & TUMPA-Lite. I believe he will help you to get OpenOCD working for WNR854T.
If you are interested, find him on FreeNode IRC server,
#openocd channel.
irc://chat.freenode.net/openocd
You can
Connect now using Freenode webchat, no IRC client needed.
Alternatively you can e-mail him. His e-mail address is
fercerpav (at) gmail.com
Please post feedback here after you solve your problem, for other users to benefit.
OpenOCD includes an easy-to-use script to facilitate mass-market devices recovery with JTAG.
For quickstart instructions run:
openocd -f tools/firmware-recovery.tcl -c firmware_help