Running with NAT as well, traffic in both directions, using connection tracking on the firewall.
Running 'tcpdump -i eth0 -n -vvvX host 54.229.214.112' on a MultiTech:
19:30:02.060869 IP (tos 0x0, ttl 64, id 2657, offset 0, flags [DF], proto UDP (17), length 40)
172.16.253.1.33474 > 54.229.214.112.1700: [udp sum ok] UDP, length 12
0x0000: 4500 0028 0a61 4000 4011 79fc ac10 fd01 E..(.a@.@.y.....
0x0010: 36e5 d670 82c2 06a4 0014 61ca 015a bc02 6..p......a..Z..
0x0020: 0080 0000 0000 a050 .......P
19:30:02.099706 IP (tos 0x0, ttl 48, id 52938, offset 0, flags [DF], proto UDP (17), length 32)
54.229.214.112.1700 > 172.16.253.1.33474: [udp sum ok] UDP, length 4
0x0000: 4500 0020 ceca 4000 3011 c59a 36e5 d670 E.....@.0...6..p
0x0010: ac10 fd01 06a4 82c2 000c 02ac 015a bc01 .............Z..
0x0020: 0000 0000 0000 0000 0000 0000 0000 ..............If you are not seeing return traffic your return IP address on the outbound interface could be wrong.