Mode A on the DMSS Tx or it won't work!
Also you sure the Rx is set to Xbus on the output and not the Aux 3 (Channel 8) output. Plug a servo into 8 to check.
You can bind the Rx and check it works-but guess its because you have B set in the Tx.

Also for failsafe operation remember to set the channels in the expander

Dave