Who is responsible for back mortgage after paperwork is signed?

[Becca Pariser]

Looking for advice:
We just closed on our first sub-to deal as end buyers (woo hoo!). We are responsible for the mortgage as of January 1 - but it's come to our attention that they stopped paying it back In October 2024. So they are two months behind. 

In this case, who is responsible for the catch-up? We already paid them a pretty sizeable entry fee ($30k) to cover some of their equity and debt in another project.