There is actually a lot written about this problem. Did you check out all the other posts in the forum? Here a few:
Have sadly no idea how the DO App platform works. But if they also create a reverse proxy for you, it is possible that it will not work because n8n needs some special settings (as described in the issues above) to work. The reason for that is Event Source which we use to push information to the editor. So you can maybe check in DO forums if there are known issue with Event Source or check with their support.
If all that does network I would advise to get a regular DO instance and follow our Server Setup guide.