I don't use the iOS app, does it
not use the 'normal' tags that WC offers? You probably use "STAT$TEMPERATURE:MAX:YESTERDAY$TEMPUNITS$" for the hi temp for yesterday. Not sure what "YHIM$" does, but I assume it is the time of the hi temp. I think that time is supplied by "STAT$TEMPERATURE:MAX:YESTERDAY:TIME$" in WC, of course.
Perhaps the dev for the iOS app also has a Manual?
Perhaps I should not attempt to answer questions about an app I don't use?