There are no additional requirements to enable saving of I/O Post/Get.
Could you tell, which firmware version did you have installed before the update and if the update was performed with Keep settings option enabled?
If that is the case, could you simply reset the device to factory defaults (in router's WebUI navigate to System -> Backup -> Restore default settings)?
In default configuration, router has one SMS rule, which sets the output to high state. This can be achieved by sending an SMS with the following syntax:
- <routers_password> ioset
- replace <routers_password> with the password you use to login to the router's WebUI.
Alternatively, you could manipulate output states using HTTP queries from a local device, if I/O Post/Get is enabled, with the following call:
To set output state to low using SMS you would need to configure additional SMS rule.
For this, login to router's WebUI, navigate to Services -> Mobile utilities -> SMS utilities. At the bottom page add a new rule by giving it an SMS text with Action set to Change I/O state. In the rule configuration window, enable it, set State option to Low and save the settings.
More information on SMS rules is provided here.
You can find more details on Input/Output Post/Get here.