General Security related Questions
The card holder will need to resubmit the transaction. The transaction was blocked.
It should take effect straight away.
However, sometimes the forms are cached and it remembers previous settings. Doing a control+refresh should work to clear the cache.
Also, double check it has not been blocked again. The IP addresses are related to Merchant Facilities, so check that it is marked as allowed for all Merchant Facilities that you want to allow it for.
If still having issues, check the Error Log - Payments2Us as that describes the error in more detail.