The CAN bus speed support is dependent on the adapter, not RealDash itself. With quick look it seems like Seeedstudio adapter does not support 83.3 kbps CAN speed 
According to the official website, the device supports CAN baud rates from 5 kbps to 1 Mbps, which includes 83.3 kbps. Which baud rate should I choose in RealDash? For CAN-C, I use 500 kbps, but for CAN-B, which works at 83.3 kbps, there is no exact option in RealDash. What should I select for CAN-B?
The above does not mean it supports 83.3, just ‘some’ speeds in between those values. From protocol documentation, these are the supported CAN speeds:
1000000
800000
500000
400000
250000
200000
125000
100000
50000
20000
10000
5000