So had help setting this up before. Identical situation, just different location. Setup exactly the way the other site is...however, I can't get the MX to come online..Confirmed manually the IP was set correctly. I'm not sure if I'm missing something.
So heres the info from comcast:
WAN usable ip addresses
So created two vlans on the ms350 (/30-999 and /29-750) put a port in access mode for each vlan, 999 going to comcast and vlan 750 going mx. Only thing i can think of is maybe the network address is wrong?