stolen Iphone, find my phone was enabled but no luck
My iphone 11 pro max was stolen last month.  The phone was passcode protected,  I had Find my phone enabled, and I went into icloud same day and marked as Lost. But I haven’t not been able to view the phone's location as its showed and continues to show as offline. I know the thief used the phone as my phone bill showed they added wifi topups to my phone the next day. (I got a new phone 2 days after with my same number.)  So why I haven’t been able to see the location of my phone despite having everything set up to do so, and knowing the thief did use the phone within 24 hours? I want to erase the device as I have personal photos etc on the phone, but if I do that then after you erase a device you cant track it, so if I do that then the phone is lost forever? what else could i have done to be able to find my phone?
iPhone 11 Pro Max, iOS 13