I’ve noticed that I’m prohibited by some apps from opening the DTTG shared folder, e.g. here you can see that 1Writer can access iCloud Drive and Working Copy, but not Yoink or DTTG:
I can’t test this just now, and don’t know the answer to your question off hand, but: does the lockout time in DTTG Settings/Security make a difference (esp. if it is currently set to immediately, you might want to try any other setting)?
It is possible in 1Writer to manually open or save files from DTTG. But sadly it is not possible to set a DTTG database or one of its groups as the default saving place. I guess that is what you tried to do.
I guess we have to contact the developer of 1Writer. It is the fastest note taking app I know about that uses the file system (unlike Drafts, sadly) and it would be awesome if it saved directly to DTTG.
At the moment I save to an iCloud folder DT indexes. Works great when you’re near a Mac.
E.g. 1Writer shares a simple folder with some files in it. DEVONthink To Go, on the other hand, uses the file provider mechanism to share a database. Unfortunately iOS only allows real, simple folders to be mounted; they simply never implemented folder sharing/mounting for file providers (at least not that we are aware of).