iOS and iPadOS do have a “WiFi Assist” feature. However, this feature works in the opposite sense in that you device will automatically try to use your mobile data plan when WiFi connectivity is poor.
Settings > Mobile Data > Wi-Fi Assist - set to ON
This extract from the iPad User Guide for iPadOS provides additional explanation:
Set cellular data use for apps and services
Go to Settings > Cellular Data, then turn Cellular Data on or off for any app (such as Maps) or service (such as Wi-Fi Assist) that can use cellular data.
If a setting is off, iPad uses only Wi-Fi for that service.
Note: Wi-Fi Assist is on by default. If Wi-Fi connectivity is poor, Wi-Fi Assist automatically switches to cellular data to boost the signal. Because you stay connected to the internet over cellular when you have a poor Wi-Fi connection, you might use more cellular data, which may incur additional charges depending on your data plan. See the Apple Support article About Wi-Fi Assist.
View or change cellular data settings on iPad (Wi-Fi + Cellular models) - Apple Support