Exactly which device type handler do you now have assigned to the plug? ( if you can post a screenshot of your devices page that would be helpful, you can crop it down to just that device if you want)
Second, just to be sure, did you paste the DTH into the Device Handler tab, not the smartapp tab? Because that “missing method” or usually means a problem in the code installation, not the DTH itself.
This might help:
My thought on this is that you might not have installed the device type handler at all, and the plug is just trying to use whatever generic DTH it paired with initially. But the device list will tell you what handler it is actually using.