I’d wanted to get this working and your post gave me the impetus to try again, as I’d been facing the same “Can’t connect to device. Check device and try again.” in the new app. I removed/re-added in the old app countless times (didn’t use a custom DTH, I just added under Marketplace -> Presence Sensors) The device would show in the new app but wasn’t clickable or usable in any automations) until… [drum roll] I reinstalled new smartthings (as was mentioned in one of the earlier posts) Force closing, clearing cache wasn’t enough, it needed a reinstall. It’s new clickable and usable in automations.
Unless something has changed since the last time I did it, uninstalling and reinstalling the app removes all of your custom UI settings. That kind of stinks as you have to reorganize all of your rooms, etc.