welcome to the community ! 
is your time keeping doc a large doc? does it show “calculating“ in the mobile app?
generally have found if the doc is more on the smallish size, adding a row to a table via a canvas button isnt an issue.
to be frank - the mobile app is at the mo lackluster at best.
The good news is there will be a mobile app roundtable later this month as per
so fingers crossed some movements in this field will be actualized this year 
hope this helps!
cheers!
Mel