If you are trying to trigger a rule from the driver sending Dry mode, then use Custom Attribute against that device, and it will offer all of the attributes the device sends. Select ThermostatMode, and then select from the values it offers.
This won't be "fixed" anytime soon in Rule Machine, as this mechanism already exists to get at any attribute/value pair for any device that is not otherwise already part of RM.
What driver is this? I was referring to the driver, not to RM. RM does not pull "supportedThermostatModes", and it appears that the driver isn't presenting them either.
I'm going to make the changes to RM to use supportedThermostatModes and supportedThermostatFanModes. A couple of interesting points: not all thermostat drivers support these attributes, and the attributes are not intrinsic in the thermostat capability (as implemented currently). That means that a default set of modes and fan modes must exist.
There is another issue for the situation when multiple thermostats are selected that have different available modes or fan modes. For this only the intersection of the modes available will be offered (least common denominator), as otherwise one could hit a thermostat with a mode it doesn't support.