Release Notes for Cisco IOS Release 15.6(1)T for the Cisco IR800 Industrial Integrated Services Routers - Cisco
The release notes above explain that the IR809 must be reset by holding the reset button when the router is being powered up.
Note that, unlike the IR829, which has a removable power plug, the IR809 power plug is screwed into the chassis, and so cannot be easily removed and reinserted.
Practically speaking, then, it is often not possible to use the reset button to reset the IR809.
As a workaround, the "clear start" command, followed by a "reload" can be used to achieve much the same effect.
