I am trying to make a use case diagram for my project, the backend is going to be made using Django rest framework and the front end using react, my question is how can i model this situation in the right way, should i model the frontend and represent the backend as an actor or the opposite, since i am thinking of making a mobile application as a second front end?
The right answer here is the Standard Answer of the Business Analyst no 1: It depends.
The question is - what do you want to model and why. Then - what is the correct tool (diagram) to do it.
The goal of the Use Case diagram is to show what functionalities a system is going to offer. Now the system can be treated as a whole, in which case you show the functionalities without depicting how the system is internally organised (this is the most common scenario and most probable the best way to use Use Case diagram in your case - but it does not show the fact of having FE and BE, note that this type of diagram isn't really best suited to do so, so keep reading).
You may also tread e.g. BE as the system itself (it can make sense especially when you're preparing headless API and really separate BE from FE; even more so when your BE and FE teams are totally separate). In such case FE will become an actor (just like e.g. other system that can interact with your BE). Obviously FE can be treated in the same way (i.e. be considered the system with BE being an actor), however usually there's less reason to do so.
Now having said that, if you want to depict the distinction between BE and FE, you should consider other types of diagrams. Keep in mind that Use Case diagram is a dynamic diagram, and the internal structure of the system is static, so obviously it should be one of the static diagrams instead. One that is dedicated to show the internal structure of the system is the Component diagram and it would most likely serve best the purpose of indicating existence of FE and BE (potentially with further level of details, e.g. existing microservices).
If on the other hand you would like to show specific technology in use, Deployment diagram might be your best shot. It allows to show the actual runtime environments, artifacts and their technologies.
Keep in mind - tying to use one type of diagram, or even worse one diagram, to show everything is usually a bad idea and a mistake often made by newbies. Be smarter than that.