I had already registered my cards in SimplyGo app but still unable to detect. I tried changing to a different card too.I started off using gpay nfc linked to my bank card for public transport without even registering in simplygo.
After a few months, i had issues getting gantry to detect my gpay nfc but i could pay for other stuff just fine. So i downloaded the simplygo app, registered the same bank card and it has been working ever since.