If a user has received a new telephone/mobile, this must be registered in the Authenticator application.

Go to 'USERS/xxxx' (xxxx=identification PID) and select SMS/EMAIL in the One time password dropdown and save this.

Afterwards, select 'Timed OTP' from authenticator app again. 

You will be redirected to the pop-up where the OTP authenticator can be activated again using the new phone number details.