When using a wired ethernet adaptor on my iPad PRO, FING will not complete a device scan.

Thanks @Marc
@IndyW

If your iPad is connected with ethernet(Wired) connection, Fing might not be able to work. Fing App works on TCP/IP model and there are L2 & L3 protocols, while physical transmission of wireless packets goes below that level at L1. 
See "Data link layer" and "Physical layer" in https://en.wikipedia.org/wiki/Network_layer. 
Scanning of devices in the user's network(wifi) requires the devices to have joined the network to recognize by the App.
Let me share some insight into how the Fing App works differently than other Apps. As Fing App is a network monitoring device which largely focuses on Wifi rather than ethernet connection as with wired connection, it is a dedicated connection which becomes hard for anyone to monitor. So, when a device(router) has made an ethernet connection to provide internet to a device, it provides a set of IP address from its pool of address after communicating with the DHCP server. Thus, for Fing App, it will be shown as those IP addresses are being used/reserved by the router rather than the device itself because those devices are talking to the internet through a dedicated connection to your Access Point(router/extender/switch). And it is responsibility of that Access Point to provide an IP address to particular device. 
 
If you can switch to wifi on your device for Internet and then run a scan, you will see those IP addresses as separate with the name and icon of the device and not the router itself.