So just to clarify here, you want to control what the users can see but you don’t know who the users are? 
how do you know if a user is coming from customer1 or customer2 in this case? Would you rely on the domains?

users need to be added in UC to see data. So somehow you need to know who they are.

in general, for sharing dashboards I would most likely embed them in a simple app. In the app, make sure that users are not relying on principal permissions but on the users own permissions.

For data control I would rely on ABAC policies with row-filters on the data to let users only see data from their own domain (assuming this is the setup)