It's because the thing you're trying to edit is the mobile interface. It cannot have DHCP at this point as it connects through a different protocol. If you go edit your Wired WAN interface, you'll see the protocol selection there, but as it is a mobile one - there is nothing to edit except connection type in Network > Mobile.
To set public IP address (if you have one given by your operator), go to Network > Mobile and set APN to Custom.
EB.