I bought a new iPhone 11 and restored the backup from my iPhone 6.
It's great except for this issue; I can't get a longer auto-lock than 5 minutes.
I spoke to an Apple support rep and we worked out it's something to do with the exchange accounts I have set up on it.
I have one Office360 exchange account and 6 AWS exchange accounts, all used for email only.
If I disable the AWS exchange accounts, for some reason I get the "Never" option back. The Office360 account doesn't affect anything.
However we weren't able to work out much beyond this.
Why could this be?