When using an external midi clock, it won‘t stay activated in the settings when restarting Drambo.
It's likely saved in the Project. Have you tried loading a Project that had external clock enabled?
I expect it's not appropriate to save it in the app settings, since your device might not be connected to the clock source when Drambo is next opened.
I am not sure if it is being saved in the project. But it happened when I open a project where this has been set and saved before. Also sometimes it does remember it, other times not. I think it is a bit buggy, hardware stayed the same and is connected. So it should save it per connected hardware, regardless of the project as those are general settings.