Does the SIM card still work in the iPhone 6? If so, the first thing I'd try is a Network reset via the settings app on your iPhone 7.
1) Launch the settings app
2) Tap, "General"
3) Tap, "Reset"
4) Tap, "Reset Network Settings"
If the SIM doesn't work at all, I'd suggest popping into your local Optus Store. We can replace your SIM card on the spot then and there (with the same mobile number). You'll want to make sure that you have 100 points of photo ID.