EOS GATEWAY problem

When we set our Eos board to Primary without connecting our RPU we get no DMX from the gateway but if we connect and set our RPU as Primary and the board as Backup we do get DMX.
If we then disconnect the RPU so the board takes over as a backup we still get DMX.
Why are we not getting any DMX from the EOS and the gateway when it is on its own as Primary?

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