iPhone 8 Plus unavailable for no reason

There's no option to bypass a lack of backup. Perhaps you've underestimated that risk. No backup = No recovery, sorry.

See these instructions, keeping in mind that if you don't have a backup all data will be lost on that device (except those that you eventually synced to iCloud):

If your device is running iOS 15.2/iPadOS 15.2 or later, AND if you have created a backup to iCloud, you don't need a computer:--> How to reset your iPhone, iPad, or iPod touch when you forgot your passcode - Apple Support

Shouldn't be this case (iOS/iPadOS earlier than 15.2), you need a computer. If you don't have a computer, borrow one from your friends/relatives/neighbors or ask for help at an Apple Store or Authorized Service Provider, then read these suggestions.

Given that to remove the existing passcode you must connect your device via USB to a computer (a Windows PC running the latest iTunes version, or a Mac, which can run iTunes or Finder, depending on the macOS installed on it), pay attention to the specific gestures that you need to perform while connecting your device to the computer.

These are the instructions to re-gain access to your iPhone/iPad:

If it is an iPhone:--> If you forgot your iPhone passcode - Apple Support

If it is an iPad:--> If you forgot your iPad passcode - Apple Support

Also, take a look at this support video about the same matter (for both iPhone and iPad):

https://www.youtube.com/watch?v=7s8-zLG9ySQ

Regards

Giulio

Bbtris wrote:

Thanks, not overly helpful.

Sorry about that.

I haven’t forgotten my passcode at all. It’s just come up like this for no reason.

You need to remove that passcode (stored inside your device), since your iPhone is now disabled/unavailable. It may have been stowed in the bag without a screen lock and, for some reason, received some input accidentally.

no idea why or how to resolve without resetting

Unfortunately, I don't know of any other methods to solve the problem.

Regards

Giulio