To add context, when the MX67/68 was designed, our cellular strategy didn't exist yet. That has changed as we developed that strategy with the MG21, then 41, and now 51. The MX75 through MX105 support POE on the WAN ports specifically for that reason.
https://documentation.meraki.com/MX/MX_Installation_Guides/MX75_Hardware_Installation_Guide
https://documentation.meraki.com/MX/MX_Installation_Guides/MX95%2F%2FMX105_Series_Hardware_Installat...
To get back to the original ask, how to power it on the MX67/68? The recommendation without POE WAN would be through a POE injector (802.3af capable), a POE switch fabric leveraging VLAN tagging on WAN 2, or a simple AC adapter. In my $0.02, a POE injector is an easy way to go and affords you additional flexibility in deployment by not being bound to an AC power plug right by the device.