TM livechat already said its private ip, not public ip. You can even ask them to check for you either through TM livechat or via calling 100 hotline staff, as long as the router is still online and connected to the internet.

Or you can check yourself by logging into the router(some people also called it modem), you can see the the router status, both IP address and Gateway column showing 100.xxx.xxx.xxx.

I only notice the change to 100.xxx.xxx.xxx about two weeks ago.

When did yours get changed to 100.xxx.xxx.xxx?