Stolen phone
If your phone is stolen and you suspend the service through your provider can the phone still be accessed through WiFi
iPhone 11 Pro Max
No, because if your iPhone is stolen then it is obviously not going to be around a known WiFi network where it can automatically log on and be active on the internet. I recommend that if more than 3 days have passed since it was stolen AND you no longer are able to track it then cancel/suspend the service to that iPhone anyway.
Here is some information on this subject ➔ If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support.
Axel F.