Download the latest iOS IPSW firmware file for your iPhone 8 Plus GSM or iPhone10,5. Since this is the latest IPSW file, the iOS version is still being signed by Apple and can be restored with iTunes or 3utools.
The Apple iPhone 8 Plus series was released on 22 Sept 2017 and comes in 64GB/128GB/256GB storage and no card slot.
The 5.5" iDevice has a RAM memory of 3GB, a 12MP rare camera and is powered by Apple's A11 Bionic (10 nm) chipset.
Device info
Identifier: iPhone10,5
Board 1
BoardConfig: D211AP
Platform: t8015
BDID: 0xC
CPID: 0x8015
How to install this IPSW file
You can install this IPSW file using 3utools following the guide here. Conversely, if you prefer using iTunes, then follow the guide here.