Internet not able to be set up?

Just Moved In
I recently moved into a duplex which has a legally registered basement and the main floor on each side. There is one fiber optic cable coming into our side of the duplex which is plugged into an ONT that has 4 ports on it. The basement tenant has a modem connected to the ONT already. I was told I cannot get my internet set up unless another cable is run into the house. Can't my modem also be plugged into the ONT?

@Noah_mc  Sign up for internet and you will get your own ont and router. Thats the neighbours internet  service.