You don’t have to “switch” to Bonjour. You can add it as a second sync method. I will “just work”. You can keep Apple’s Cloud Kit if you want and hope/wait for it to work, but meantime, after setting up Bonjour and being in a situation where it works (both devices on local network), it will just work.
To set up Bonjour, rather than me repeating again here the instructions, please see the outstanding “DEVONthink Handbook”, page 60 of the current version (3.8.5). Supplemental information at Bonjour Simplified