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.