You will not be able to access a whole structure that contains any of the TIMER SDTs like IEC_TIMER, IEC_LTIMER, TON_TIME, or TON_LTIME.
These types contain hidden members necessary to their function that are not accessible from OPC UA or to be touched by anything else in the system.
Including one of these in your UDT implicitly creates what they refer to informally as a “Swiss cheese structure”. The same effect can be seen by making any member of a structure not accessible from HMI / OPC UA. Once you do this the structure definition disappears and it is not accessible as a whole.
Their suggestions is to make your own timer UDT with the members you need and copy/update those members from the user program.