I am also interested in this, and perhaps I could help. Seems like the dtr radios are based on the iden 900MHz of Nextel. Perhaps one of these T6737A iden 900MHz quad base transceiver systems on eBay could be repurposed to significantly extend the range of the DTR systems on private internet networks?
I’m a wireless / network / systems architect (read: hobbyist) that has been interested in these dtr/iden PTT systems for some time now.