Reminder notifications on mobile devices (Android/iOS))
This topic has been covered quite regularly already. Since OP asks about specific pitfalls, they seem to have done some basic research. The question seems to ask for someone to summarize all available information in one post, which is not what Stack Overflow is for.
In my opinionIf they did try things already and got stuck, this should just be a normal "how to do ""why doesn't my work?" question, where OP shows what they tried. Unless there is a specific problem already, it is not a discussion topic.