Reinstalling the wemo app usually fixes this. You can also use alexa and ifttt with no problem.

Product Review Wemo Mini Wifi Smart Plug Smart Home Devices
Wemo mini not detected. If you have unplugged a wemo device the wemo app may still have it registered and may detect it. Wemo device not detected when linking to your amazon alexa amazon alexa allows you to quickly and easily control your wemo devices just by using your voice. In case the wemo app unexpectedly freezes or becomes unresponsive force close it and then open it again. Check if your wemo device is unplugged or removed from the wi fi. If you have unplugged a wemo device the wemo app may still have it registered and will look for it. Though the app is saying not detected wemo is connected to your home wi fi network and it keeps working on a schedule timer etc.
If you have multiple homes with wemo devices in them you will only see one set at a time. Your wemo device should be plugged in and connected to the same wi fi where you manage your wemo devices. In cases where your wemo device is not detected when linking to alexa try the troubleshooting tips below. To know how you can link your wemo devices to your alexa click here. Check if the inherited houses wi fi is registered on the wemo app since that is also where your wemo mini is currently located. Lets try to disable and enable it but make sure youre at the inherited house and your phone is connected to the wi fi there.
For those who use wemo app to control devices i suggest to turn off wi fi on your smartphone. This will be grayed out in the wemo app.