I lost my iphone 7 plus

You can't find it via IMEI or SN number. Did you have Find my iPhone enabled on your phone. If you did and whoever has it turns it on, you can see its location and even disable it. You should report your phone as lost/stolen to the police and follow the steps in this link: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Physically search for it as you would any lost item.

It isn’t possible to find it through the serial number, and doing so through the IMEI is expensive.

(153730)

The vast majority of lost or stolen iPhones are never recovered, sadly. Do follow the instructions in the link I provided.