There is no way to reach Split without reservations. However, you could do this leg by night train and save yourself one night of accommodation. You travel at night, which saves you time to explore by day as well. Dubrovnik is not connected to the railway network, so you need to take a bus from Split. These buses are not covered by Interrail.

As @Mattheas  said you can´t go without a reservation to Split but depending on the Route you take you doesn´t have to pay much :)

For example take the direct EC from Vienna to Zagreb and change here for the domestic Nighttrain to Split depending on the comfort level you just have to pay from 1€ for a seat up to 40€ for a private sleeper compartment Here Reservation Prices Zagreb -Split

On top you can catch the twice weekly direct Nightrain from Bratislava via Vienna - Graz to split with Reservation fee´s starting from 14€ for a seat

Thank you so much for your answers!

Since I’m new to this, I’m still getting used to search the trains/connections, what do you think is the best way to search/organize things in order to pay the less possible? 
Sorry if it’s a complicated question and thank you! 

That's a difficult question for an easy answer hehe. As a general rule of thumb, in countries with reservations, the more borders you cross and/or the more premium services, you pay more in reservation fees. This also applies to night trains.

For example, the direct night train Vienna - Split is operated by Austrian Railways, which is more expensive than the night train Zagreb - Split, which is operated by Croatian Railways. 

In some countries and on some routes you can avoid reservation fees (Italy for example), while in other places this is not possible (Spain, or the Zagreb - Split route).

What I often do is to break up the journey and see if there are trains with cheaper or no reservation fees (like the example of Seewulf), or even check no reservations in the app for the whole journey or for part of the journey. This way you'll find out that you won't need a reservation for Vienna - Zagreb or Vienna - Ljubljana.

I use mostly different Railplanners for example (DB Navigator or Scotty) and use the Railplanner App just in case iam not sure if a train needs a reservation or not. Or if the train is covered or not.

To keep reservations small you can look in the Reservation page of interrail as they have there a Page with average Reservation fee´s In General the West (Spain,France, Italy) have the most expensive reservations with going more and more east reservation are often even not needed anymore or are quite cheap from 1€-5€.  (Exceptions of Nighttrains as here the Price depends on the ComfortLevel)

Most of my knowledge i learned on my trips and from other travelers that i met on the way or for example in the olf Facebook Group. Now i happy to share it with others and try to give suggestions and advices how to handle everything.

I started often with a list of Cities and spots i wanted to visit afterwards check traveltimes between them and then sorted them in a logical order.

Thank you everyone!