Unlock Optus Samsung Galaxy S3 Mini

I have a Samsung Galaxy S3 mini that I resurrected recently. I bought a Woolworths mobile sim forgetting that my S3 mini is locked to Optus. When I bought the phone I never used an Optus prepaid plan, I straight away used a reseller (I think it was Woolworths mobile back when they used Optus network).

I tried unlocking using the official Optus unlock page, but it doesn't recognise the details and days it's not an optus phone. I think it's because I never registered my details directly with Optus using that phone.

Any suggestions for how to unlock the phone for cheap? I already topped up with Woolworths credit, so hoping not to have to waste that credit and go to an optus reseller.

Any help appreciated.