iPhone 7 Plus stuck in recovery mode and can't restore/update even in DFU after downgrading from iOS 11
I would avoid DFU for figuring out what is wrong - it's kind of a last ditch effort to revive a device without a lot of diagnostics or error reporting. Since Apple restricts who can receive betas and asks them to communicate with users on the various seed and beta discussion groups - many people here will decline to engage on beta discussions so you might get better answers on Apple's forums devoted to the beta programs.

Also - since you know the firmware is on iOS 11 at this point, why not try reinstalling iOS 11 again to see if the device is still working?

Foregoing that, what happens when you plug the device into iTunes and attempt a restore? 

That operation is designed to log error conditions, statistics, reasons and many items to log files. From there you would benefit from being able to reach out to Apple support and also have a large audience with experience in restore errors. It could be an unlikely hardware failure or a likely firmware issue where you are now committed to running the beta pending some changes to the software either on the iTunes side or on the iOS side to allow you to go backwards in code version.

If other people have this issue, I would suggest asking a new question with specific details like - what version of iTunes and what OS (Mac or windows) and what specific error message you get before you try DFU. As you have acknowledged in the post - recovering from DFU failure is usually with Apple Certified technicians and/ or hardware replacement assuming your PC/Mac and iTunes isn't the cause of the restore issue.