I have tried out different apps such as Nirvana, but Doit.im seems to be the best still.
Unfortunately their team don't reply since 2019, and they said that their major developer was sick.
Lets cheer for them to get back operational since other apps are far from as good as Doit.im for GTD.
This way i found a way to get back using it in my Samsung Note 9.
1. For syncing, I click on Menu>C>Account Info>Logout>Sync and Logout
2. Monitor in Menu>C>Sync Log if the log is being updated
I was having a problem in my account while paying the Pro version through google play although it wasn't changing to Pro.
Then I canceled Pro inscription on Google Play and made it through: Menu>C>Account Info>Renew.
Interestingly there are two ways to log on: through Gmail, or through registered Email and Password.
Once I logged out and logged on again through Email+Password my Pro version got back working.
I will try to use Doit.im as long as I can, hopping they get back updating the software.
Hope it helps.