This may sound silly but have you had a chance to double check if the 4G back-up SIM is already inserted into the modem? The 4G back-up SIM usually comes pre-inserted in the modem and would have already been activated, however there are times when it may arrive in a separate delivery.
If you are still having troubles with accessing the 4G back-up service, we would recommend contacting our Technical Support teams via the My Optus app for further assistance.
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