First - the solder joints to the heater connections on the RAMBO boards are a known failure point: TAZ 6 Heated Bed Not Heating - Perplexing Puzzle - [EDIT - SOLVED!! Heater pin melted on RAMBo!] - #21 by kmanley57
If it passes that inspection, continue:
What temp does the display say the bed is when you turn the machine on? If you get 0° for it - could be wiring, harness, or thermistor. Plug the new bed into the harness and see if you get the same 0°. If you get a temp reading with the other bed, swap beds. If it’s still 0? look at wiring from the harness to the board. Check for continuity along both thermistor wires.
If the temp display is within a few degrees of the nozzle’s temp, the thermistor should be good, but heater could be bad. If you manually tell it to heat and it doesn’t heat up, could be wiring to the bed heater, the bed heater, or the control board. Follow wiring for continuity back to the board, if it’s all good, replace the board.