I think this is a pretty clear situation. You have been hired as a C# backend developer who could use some knowledge in frontend technologies, and you ended up doing mostly frontend, so you're not working on what you've been promised. This is a problem, and you are right to bring this up to your management.
People tend to forget a very important thing: you have the right to be unsatisfied by your job. If you have been lied to about what your job would be, you have the right to bring it up. Of course, it could be that the people who expressed the need for a C# developer overlooked the need for a frontend developer, but that does not matter to you. If you are not working on what you are there for, you need to reconsider working there. It's a two-way street, really, companies lay off people if they're not happy with their performance, if you're not happy with the company you're working for, lay them off.
Now, I'm not saying you should quit right away. You should always try to resolve issues in the simplest way possible. The first thing I would do is talking with your team (if you have one), and suggest they might need a frontend developer to handle the work you've been doing recently. If you're working alone, you can skip the rest of this paragraph. You can either first bring this up casually (recommended), or Schedule an appointment with your team lead to discuss it. It needs to be a discussion, and not a rant: no manager/team leader would take it positively if you just came to complain. Just explain the situation and try to suggest that you would be more efficient if you could work on the backend tasks you were hired for, and thus that having someone else working on frontend would help the team's productivity.
If this doesn't work, or if you're not part of a team, you can indeed bring this up to management. Again, it's not about telling them you're unhappy with what you're doing, it's about suggesting an idea to improve overall productivity. You can try to explain that a developer specialized in frontend would be more efficient, and allow you to work on backend tasks, on which you would perform better yourself. To maximize the chances of this working, you need to make the manager feel like he's not doing it to accommodate you, but rather to improve productivity.
At this point, if things don't change quickly, I suggest you start looking for another job. It means they do not care about what you want to work on, and they will just have you work on the tasks they need. You can probably find a place where your expertise will be better used, and where you will get more respect from management. Plus, you're a software developer, and that means you shouldn't have too much trouble getting a job elsewhere.
7-8 months is already a very long time. I don't feel like your company has any different plans for you, but you should make sure of that by yourself.