Where can I find the IMEI number of my lost iPhone?

go here

Find the serial number, EID, or IMEI on your iPhone, iPad, or iPod touch - Apple Support

and scroll down to the sections around

"Use the Finder, Apple Devices, or iTunes"

and maybe

"Use your Apple Account"

and the likes

If you already tried (to no avail) all the steps provided in this support article:--> Find the serial number or IMEI on your iPhone, iPad, or iPod touch - Apple Support , I suggest that you try contacting your cellular carrier and ask them.

Regards

Giulio