I haven't tried this on iPhone yet, but on android you can share Wi-Fi connection. Many times I've connected phone to hotel Wi-Fi and then chromecast to my phone.
Sadly, we're both on iPhone and you cannot share Wi-Fi like that on iPhone :(
This was a bummer just the other day where my iPhone somehow (and I still don't know how) auto-signed into an amazingly fast wifi AP managed by my cell provider, but I was stuck working on crappy guest wifi on my laptop and could not get the laptop onto the better wifi.
