Many times when i was installing "Theos" on my iOS device everything went well till the last step which is "apt-get update". When i ran the command, it would always give me an error "W: GPG error: http://coredev.nl iphone Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY" as shown in the below screenshot.
After much googling, i found this link which explains the solution to this issue. Though the scenario in the link is different than mine, however it seemed to work well for me :)
The solution was, navigate to the sources.list.d folder and add the "public key" to that folder. This can be done wither by using "lynx" or by using "wget".
A shown in the above screenshot, apt-get works well after the above steps were followed.