Connecting Freematics OBD-II UART Adapter V2.1 to MEGA - no data transfer

I am at a loss as to what pins to connect the unit's RX TX pins on a MEGA.  

I connected it to SERIAL1 and it does not work.  No data flow.

Docs say that on MEGA it needs to be serial1 (pin 18, 19)  but not work.