TTL Long Distance Transmission Alternatives
You could probably use RS-485 for the task if you don't want to use this modems. It would help to post more information about what exactly this modems are.

RS485 can help you as you could convert TTL signals into a twisted pair driven by a RS485 transceiver, as it works up to 4000ft.

I could not request more information before making up an answer because I can't put up comments.