Hi Charles_abc,
This is a tricky situation and it would be best to speak with the Credit Ombudsman.
With my thinking, as you have signed the loan documents, you agreed for RAMS to fund the loan. So once the funds were sent to the account, this completed the transaction.
This is not the answer you want to hear but if you want to go back to NAB, you will need to wait 6 months and refinance them back.
Call the Credit Ombudsman to discuss your situation, as this is super uncommon.
OR you can request RAMS to match the NAB rate that they offered.
Good Luck!