I have no idea where you got your information, but let's start at the very beginning citing reputable sources as we go.

This youtube video shows the process for enrolling a new device in WhatsApp. Since WhatsApp is built on the same cryptography as the Signal app -- called the Signal Protocol (source), let's assume they do the same stuff inside (Signal is open source while WhatsApp is owned by Facebook, so easier to know what's going on in Signal). Enrollment Steps:

1. Provide your mobile phone number.
2. The server sends you an SMS with a one-time activation code.
3. The device creates a cryptographic public key.
4. The server registers this public key against your phone number in a big database.

You are now enrolled. When your friend wants to send you an encrypted message, they ask the server for the public key(s) registered to your phone number, and they can start encrypting for you.

Nowhere in there does it use your IP or MAC addresses, so whichever friend told you that is probably not a reliable source for tech info.

It does however assume that you are the only person who can receive SMSes at your phone number, so if you lose your SIM card, or are paranoid about nation-states, then it's possible for someone to register a second device under your phone number. You'd notice pretty quick though because you'd start seeing messages that you didn't send.

I have no idea where you got your information, but let's start at the very beginning citing reputable sources as we go.

This youtube video shows the process for enrolling a new device in WhatsApp. Since WhatsApp is built on the same cryptography as the Signal app -- called the Signal Protocol (source), let's assume they do the same stuff inside (Signal is open source while WhatsApp is owned by Facebook, so easier to know what's going on in Signal). 

Enrollment Steps:

1. Provide your mobile phone number.
2. The server sends you an SMS with a one-time activation code.
3. The device creates a cryptographic public key.
4. The server registers this public key against your phone number in a big database.

You are now enrolled. When your friend wants to send you an encrypted message, they ask the server for the public key(s) registered to your phone number, and they can start encrypting for you.

I've never used WhatsApp, but Signal lets you link as many devices as you want to your phone number. But there is a menu to unlink devices.

Nowhere in there does it use your IP or MAC addresses, so whichever friend told you that is probably not a reliable source for tech info.

It does however assume that you are the only person who can receive SMSes at your phone number, so if you lose your SIM card, or are paranoid about nation-states, then it's possible for someone to register a second device under your phone number, and then (if it works the same as Signal) unlink your first device. So don't lose your SIM card.