I need to make a mobile app where the client described the problem, why they need the app, the idea about it, but nothing more. I already understand what kind of features it should contain to solve the problems that the end user experiences without the app (from my experience), but keeping in mind the point, that we need to understand the target user, what would be the best method for this if I can't get to the users? How can I gather this information, so I can validate my assumptions?
I would always advocate for talking to actual humans who are going to be the users of this app or service, but if you can't there are online tools for remote testing some basic usability concerns. I personally use usabilityhub for that kind of work. You can try to use social media or something similar to recruit the target audience. If not some research is better than nothing, so talk to anyone you can find; family, friends or people in a coffee shop.
Ask them some questions about what they think this app is for, what do they expect to happen when they do X or Y. What do specific areas mean to them.