I've recently deployed a fair ammount of MG21E's along with the MX68.
Thankfully the MX68 comes with two built-in PoE ports, however the MG's naturally would like to be connected to the WAN1/2 ports. Hence their mode of operation 😉 But at the current state of things - thats not so easy.
So I came up with the following workaround/solution:
MG port1 - MX port 11 (for PoE)
MG port2 - MX port wan1 (for uplink)
MX port 11 is configured as a trunk, under native VLAN "drop untagged traffic", allowed vlan: you need to add something. With this port11 provides PoE power to the MG, but the MG linknet comes nowhere.
The actual wan connection is done over MX wan1 and MG port2.
Works for now, but I've issued a "feature request" for moving wan1 and wan2 to the PoE ports on the MX68 (and similar models).
I've also deployed two MGs (multiple carriers) on one MX using the same method and that also works. Using MX wan2 and port 12.