[bobsky]

For those looking for it, go to Settings > Cellular. Then scroll down and tap the slider for Wi-Fi Assist.

https://support.apple.com/en-us/HT205296

[kiliankoe]

I have that enabled and the setting states that ~70MB have been downloaded in this way, but I don't know over which period and it also never does "feel" like it's working.

[CodeWriter23]

Watch some Netflix over semi-reliable WiFi and I assure you will feel it working when you get dinged for a data plan overage. True story.

[m_eiman]

IIRC it's only used for certain system services like Siri etc.