Asterisk PJSIP Realtime - No voice between two endpoints

Hello,
I have successfully configured Asterisk 13.4.0 with PJSIP realtime.
All things are configured properly as per document in this link.
https://wiki.asterisk.org/wiki/display/AST/Setting+up+PJSIP+Realtime

pjsip enpoints I added to mysql db as per document and are loaded also as per below.
But while calling between two endpoints through zoiper, no voice coming. My server is on Amazon.
I had applied [transport-udp-nat] settings through pjsip.conf and it is as below,

[transport-udp-nat]
type=transport
protocol=udp
bind=0.0.0.0
local_net=192.0.2.0/24
;local_net=172.31.30.10/20
external_media_address=54.200.x.x
external_signaling_address=54.200.x.x

ip-172-31-30-10*CLI> pjsip show endpoints

Endpoint: <Endpoint/CID…> <State…> <Channels.>
I/OAuth: <AuthId/UserName…>
Aor: <Aor…>
Contact: <Aor/ContactUri…> <Status…> <RTT(ms)…>
Transport: <TransportId…> <BindAddress…>
Identify: <Identify/Endpoint…>
Match: <ip/cidr…>
Channel: <ChannelId…> <State…> <Time(sec)>
Exten: <DialedExten…> CLCID: <ConnectedLineCID…>

Endpoint: 101/101 Unavailable 0 of inf
InAuth: 101/101
Aor: 101 1
Transport: transport-udp-nat udp 0 0 0.0.0.0:5060

Endpoint: 102/102 Unavailable 0 of inf
InAuth: 102/102
Aor: 102 1
Transport: transport-udp-nat udp 0 0 0.0.0.0:5060

Endpoint: 103/103 Unavailable 0 of inf
InAuth: 103/103
Aor: 103 1
Transport: transport-udp-nat udp 0 0 0.0.0.0:5060

Endpoint: 104/104 Unavailable 0 of inf
InAuth: 104/104
Aor: 104 1
Transport: transport-udp-nat udp 0 0 0.0.0.0:5060

Endpoint: 105/105 Unavailable 0 of inf
InAuth: 105/105
Aor: 105 1
Transport: transport-udp-nat udp 0 0 0.0.0.0:5060

Endpoint: 106/106 Unavailable 0 of inf
InAuth: 106/106
Aor: 106 1
Transport: transport-udp-nat udp 0 0 0.0.0.0:5060

Endpoint: 107/107 Not in use 0 of inf
InAuth: 107/107
Aor: 107 1
Contact: 107/sip:107@182.70.65.126:18956;ob Avail 1379.240
Transport: transport-udp-nat udp 0 0 0.0.0.0:5060

Endpoint: 108/108 Not in use 0 of inf
InAuth: 108/108
Aor: 108 1
Contact: 108/sip:108@182.70.65.126:18486;rinstance=f6058 Avail 316.040
Transport: transport-udp-nat udp 0 0 0.0.0.0:5060

Hey, I found one solution, we can add following under [transport-udp-nat] section in pjsip.conf and reload,
media_address=54.200.x.x
This works for me some days.

But one day all endpoints stopped working, don’t know why. when doing pjsip show endpoints it gives,
Endpoint: 107/107 Unavailable 0 of inf
InAuth: 107/107
Aor: 107 1

here not showing transport object, anyone encountered this please help.