When I connected another router to the WAN port of the MH7021 the default IP address of the MH7021 automatically changed from 192.168.1.1 to 192.168.100.1? Why?
The MH7021 is smart enough to avoid IP address conflicts. If the device that you connect to the WAN port of the MH7021 is in the same subnet of 192.168.1.x, it will automatically change its own LAN IP to 192.168.100.1 to avoid any address conflicts.
An important note:
To get it back to the default settings of 192.168.1.1, you will need to remove the WAN connection first and then use a paper clip to push the factory reset button for 8 seconds.