iPhone 7 Plus stuck at Recovery Mode after downgrading from iOS 11
Solved it with the following steps


Manually download the IPSW file corresponding to your iPhone model via the links found on Reddit.
Connect iPhone to Mac/PC
In iTunes, select the phone and click on "Upgrade" with either Opt/Alt (Mac) or Shift (PC) pressed, then select the IPSW file you've just downloaded
Installation will start and you will be back at iOS 11.