Run a coax directly from the cable box to the back TV through a window and verify that the TV is OK. Next use a wire tracer or DVM and trace and verify the coax location and it's connection. Ditto from the back TV coax. When you find the other end of the back TV coax connect it to a signal and see if the back TV works. Since OTA is OK on the front TV you could use it initially and get that portion working.
If you have a powered antenna then the connection to the back TV will be after the power panel. Somewhere there will be a splitter for the back TV. Pull that panel out and look for a second coax and behind the front TV connection.
The problem could be almost anything: bad coax, bad connector, not connected, bad splitter, switch box and more.