So lets see if it is 0mn then:
put a tape in the device and then
type
mt -t /dev/rmt/0mn offl
# It should eject the tape...
# tape devices are in /dev/rmt/
# 0m should be the first tape then 1m etc...
# 0mn : the n is for no rewind but mt command works only if norewind device...

If mt succeeded if putting the tape offline (ejected)
then you know what device to use: /dev/rmt/0m or 0mn
Is there a fbackup/frecover command on your station? (I deal with servers...)
You could try to use tar command
But most important since you OS is legacy I would check and see if make_recorevery is installed:
/opt/ignite/bin/make_recovery
If you have it do a make_recovery -Av