Help Unlocking Australian S4 to use in UK

Hi, My dad posted me his old S4 from Australia. It is on Telstra.
I am on GiffGaff and when I inserted my SIM card it doesn't pick up a network.

I found some instructions on how to unlock it but they aren't working.
(redmondpie. com / how-to-sim-unlock-samsung-galaxy-s4-gt-i9505-for-free/)

Can anyone please give me some advice on what to do? The phone is not rooted or anything yet and I might need some basic instructions if I do need to root it..

Many thanks in advance!