You need a true 75 Ohms cable for SPDIF over coax.
Best is to terminate it with true 75 Ohm connectors.
If you don’t you still have an impedance mismatch.
RCA connecters are not 75 Ohm.
That is the reason why BNC connectors are recommended.
Some go as far as to change the RCA receptacle on the DAC by a BNC and buy a cable terminated with BNC.
Be careful because many BNC connectors are 50 instead of 75 Ohm!
