SOLID design principles on MEAN application

me and my team (good programmers but bad in architecting) have started working on a MEAN application with following ER diagram. http://ift.tt/2v7VM0U

We have written REST apis for following use-cases so far. We see our whole code violates all of the SOLID principles, we have written Mongoose models and router classes without any principles or patterns

1. user registration
2. user fills the initial info (used for screening the prospect, once screening is done this info is never used) via a link send in email. (user is not allowed to login yet)
3. user pays the registration fee via link from email(sent when planner changes the stage of the user), now user is allowed to login.
4. other use cases like forgot password etc.
5. Upon login user fills bunch of forms like Profile(Profile, Spouse, Dependents), assets(not modelled yet), liabilities (not modelled yet) etc
6. Similarly Chief Planner or Super admin have login use, Invite team member use case, change prospect to client use case.

We want to model this with a good principles, we followed Robert C Martin's - Clean Architecture and Design from https://www.youtube.com/watch?v=Nsjsiz2A9mg, we are still confused, can you help us model this with SOLID/Domain Driven.