You could create a BVI group out of Gi0/0 and Gig0/1 and put them into the same layer 2 domain.  Then probably use a /29, so the two MX WAN interfaces  are in the same subnet as the 1921.
 
Personally, I would buy a 4 port Ethernet module for the router and use that instead.  Much simpler.  You probably need the part EHWIC-4ESG.
 
Note that if you only have Ethernet connections then it will be cheaper to buy a Cisco 891F - and the 891F will probably be faster than a 1921.