Reinforcing @chrillek, delete/reinstall DEVONthink ToGo will not fix anything. You are missing databases because you’ve not made a successful sync. Go back to where the syncs did happen (Mac?) and Verify there. And yes, probably good to pick a different sync method. Bonjour found by many to be very fast and reliable (unlike Apple’s services, both iCloud and CloudKit). See the DEVONthink Manual for how to setup Bonjour, and supplement those instructions with Bonjour Simplified
I use Dropbox (free version) synching one “works in progress” database to iPhone and more databases (with shallow sync) on iPad which has more space, and Bonjour with both.