DMZ question

Hi everyone.

I'm trying to setup a DMZ in my home lab. By reading online I can see that a DMZ is a restricted space where the device in the DMZ can not communicate with the internal network so if the device on the DMZ gets hacked the hacker cannot get into the internal network. When I put my device(Windows web server ) in the DMZ it is still able to ping and talk to devices in the internal network. Is this normal ? I have a tp link router .

If the DMZ device can ping my internal network how is the router protecting the DMZ device and keeping it in its own isolated corner ?

Thanks