Are you plugging directly into the MX, or something else (like a switch) and then the MX?
Tried both ways, either with a trunk port to a switch, or with one of the ports configured for access. In both instances devices are issued an IP and connectivity to the MX is fine, the issue is getting beyond the MX.
If something else, are you using VLANs on the MX? Have you got a matching VLAN config on the switch?
See above, if there were mismatching VLANS, clients wouldn't be able to ping the MX.
Can clients ping the MX default gateway?
Yes
Can clients ping the Internet by IP address (such as ping 8.8.8.8)?
Nope