Hi,
2.6 firmware will have modem mode available for selection in RS232 page. Release of 2.6 firmware for TRB devices should come out in 1 or 2 weeks.
Going back to older firmware then 2.X will require fastboot flashing, which if it's done incorrectly can brick your device. So I would really suggest waiting for those few weeks while the firmware is prepared for release.
EB.