WiFi Assist will, as indicated within device Settings, “Automatically use mobile data when Wi-Fi connectivity is poor”.
As such, your WiFi+Cellular model iPad (or iPhone) will use the WiFi connection whilst it is available - but will use Cellular when WiFi connectivity is poor. It follows that iPad will use the Cellular connection where WiFi is not available.
Put another way...
Your iPad will always favour WiFi where available - augmenting poor WiFi with Cellular where needed - and will switch to Cellular where WiFi is not available.
This is obviously contingent upon you having a WiFi+Cellular model of iPad with an active Cellular data plan - within an area with Cellular coverage.
I hope this explanation and clarification is helpful.