Can you clairfy what you mean when you say "I already changed the WAN 1 configuration to reflect the IP4 settings on the cellular router?" If there was something connected to the MX64, maybe you have a bad arp table, have you tried rebooting the MX64?
If the cellular router just hands off DHCP to the device behind it, then we shouldn't need any configuration of the WAN 1 interface - this should be about as plug and play as plugging in a ethernet connection from a standard SP L3 demark like a DSL Modem.