Hi D,
There is a know DJI Go/Litchi issue with connecting to iOS 11.1.x. As msinger suggested, upgrading to 11.2.1 has solved problems for most--it did for me.
However, one thing I am no longer able to do consistently with most recent releases of DJI Go 4 and iOS 11.x is be able to toggle between Litchi and DJI Go 4. Toggling back and forth most likely will result in both applications no longer connecting to your aircraft. If you find yourself in this situation you will need to either re-boot the aircraft or your iOS device to be able to again connect to the aircraft.
Let us know if his works for you.
Merry Christmas,
Donald Barar