How can I replace a lost phone

In order for Apple to replace the device you would of had to purchase AppleCare+ with Theft and Loss. I would suggest seeing if you have coverage from your carrier.

check out https://support.apple.com/en-us/HT201472 .. This will help put your device in lost mode.

You might want to check if you have any other insurance which covers loss, otherwise, you will have to purchase a new device at the regular price.

You might also find this helpful regarding your lost phone

If your iPhone, iPad or iPod touch is lost or stolen – Apple Support

Unless you have some form of insurance, you buy a new phone.