My Xperia Z Ultra Screen dpi setting change doesn't stick after reboot

I flashed the rooted 5.1.1 firmware, and the developer made the dpi really small so that it looks like a tablet. However, I want everything to be bigger. I want to change from 320 to 400. I tried Dpi changer app on play store, and also tried build prop editor app to change manually, but after rebooting, the setting doesn't stick. It's back to 320.

Could someone please let me know why this is happening? Any idea how I can change to 400?

Thank you so much!