iphone 8 plus "no Network

'Hi there,

I have a iphone 8 plus that I didn't use for a while - It was a mtn contract previously. I have purchased a mtn pre pay sim card to use it again. and now it says "No Network"

I Have researched and watched so many videos on what to do regarding the setting, but nothing seems to work.

I have run a diagnostic on itunes and this is what it said.

connection attempt to firmware update server was unsuccessful.

The network connection timed out.

can anyone assist with what to do next?