I had the same exact problem, so I had to make a little system using the UTC time plugin (It's included in the project), where when the app is quit, it saves the current time and when you enter the game again it compares the time, and calculates stuff. I have that system setup in the file, it's pretty easy, just analyze it a bit and I'm sure you'll be able to take it and adjust it to your needs. I hope it helps.
(I have marked two comments in Red, and in between them is the code you need)...
What the code does: Basically when the Game Suspends it saves the current time into the variables and then when you enter the game starts On Start Of Layout it calculates the time passed in Seconds and puts it into a variable called timePassed. Then you can use timePassed to calculate the size of the reward while you were offline or if a day has passed (timePassed > 86400) (86,400 seconds = 24 Hours) give the player the daily reward.
drive.google.com/file/d/18rKyFUxl3f5NvXrhdVn4UnAPgeyyhhKM/view