Looks like there was a recent change to add MTCAP-0.1 hw support to the utility. It will not work for MTCAP-0.0, perhaps you have a early rev?
$ mts-io-sysfs show hw-version
MTCAP-0.1
28 was an intermediate release for the v1.5 cards and AP units.
The spectral scan utility may be different from 31.
Other LoRa rx/tx operations are same.
We use 31 and 33 in our cards and AP units.
31 – spectral scan
33 – listen-before-talk