Phone is locked with an impossible to enter password As it appeared, the on-screen keyboard for entering the unlock password is missing English layout, although it for sure was present when the password has been created.
Now the situation appears really bad: 


the phone (TP-LINK Neffos C9A, Android 8.1 NFUI 8.0) is locked,
the password is not possible to enter because of missing keyboard layout,
option for adding keyboard languages is not available, 
there is no option "Forgotten password" and 
Android Device Manager responds "Can't reach device" with either WiFi or 3G connection.


How to unlock the phone in such situation?
In case there is a problem with the on-screen keyboard most devices support to use an USB keyboard connected via USB On-The-Go (OTG) adapter to the phone - if the phone supports USB OTG at all.

The simpler an USB keyboard is the more likely it will work - keyboards with integrated USB hub and/or readers for smartcard or sd-card often cause problems and may not work.

Note that the USB keyboard works like a regular keyboard (not like the on-screen keyboard): key combinations like have be pressed at the same time, not one after another.