It seems that the problem was due to selection of the wrong context ID. with context ID=3 it is able to acquire the IPv6 address. I am still facing problem on ping6 to any IPv6 address.
eth3 Link encap:Ethernet HWaddr 52:53:AA:56:FD:08
inet addr:100.103.33.157 Bcast:100.103.33.159 Mask:255.255.255.252
inet6 addr: 2600:1012:b152:a173:5053:aaff:fe56:fd08/64 Scope:Global
inet6 addr: fe80::5053:aaff:fe56:fd08/64 Scope:Link
UP BROADCAST RUNNING NOARP MULTICAST MTU:1500 Metric:1
RX packets:21 errors:0 dropped:0 overruns:0 frame:0
TX packets:46 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:3210 (3.1 KiB) TX bytes:4453 (4.3 KiB)
ethtool -i eth3
driver: GobiNet
version: 22-Aug-2005
firmware-version: GobiNet Ethernet Device
bus-info: usb-bcm-ehci-1
supports-statistics: no
supports-test: no
supports-eeprom-access: no
supports-register-dump: no
supports-priv-flags: no
OK
AT!CUSTOM?
!CUSTOM:
GPSENABLE 0x01
GPSLPM 0x01
IPV6ENABLE 0x01
UIM2ENABLE 0x01
SIMLPM 0x01
USBSERIALENABLE 0x01
DHCPRELAYENABLE 0x01
OK
AT!IMPREF?
!IMPREF:
preferred fw version: 02.24.05.06
preferred carrier name: VERIZON
preferred config name: VERIZON_002.034_000
current fw version: 02.24.05.06
current carrier name: VERIZON
current config name: VERIZON_002.034_000
OK
AT!GSTATUS?
!GSTATUS:
Current Time: 536 Temperature: 35
Reset Counter: 1 Mode: ONLINE
System mode: LTE PS state: Attached
LTE band: B4 LTE bw: 20 MHz
LTE Rx chan: 2100 LTE Tx chan: 20100
LTE CA state: NOT ASSIGNED
EMM state: Registered Normal Service
RRC state: RRC Idle
IMS reg state: No Srv IMS mode: Normal
PCC RxM RSSI: -70 RSRP (dBm): -100
PCC RxD RSSI: -69 RSRP (dBm): -100
Tx Power: 0 TAC: 2F02 (12034)
RSRQ (dB): -9.3 Cell ID: 054C8E20 (88903200)
SINR (dB): 11.4
OK
AT+CGDCONT?
+CGDCONT: 1,“IP”,“”,“0.0.0.0”,0,0,0,0
+CGDCONT: 2,“IP”,“”,“0.0.0.0”,0,0,0,0
+CGDCONT: 3,“IPV4V6”,“vzwinternet”,“0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0”,0,0,0,0
+CGDCONT: 4,“IPV4V6”,“vzwapp”,“0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0”,0,0,0,0
+CGDCONT: 5,“IPV4V6”,“vzw800”,“0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0”,0,0,0,0
+CGDCONT: 6,“IPV4V6”,“vzwclass6”,“0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0”,0,0,0,0
OK
AT!SCACT?
!SCACT: 1,0
!SCACT: 2,0
!SCACT: 3,1
!SCACT: 4,0
!SCACT: 5,0
!SCACT: 6,0
OK
AT+CGPADDR
+CGPADDR: 1,0.0.0.0
+CGPADDR: 2,0.0.0.0
+CGPADDR: 3,100.103.33.157,38.0.16.18.177.82.161.115.0.0.0.21.203.11.145.1
+CGPADDR: 4,0.0.0.0,0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0
+CGPADDR: 5,0.0.0.0,0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0
+CGPADDR: 6,0.0.0.0,0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0
OK
AT+CGCONTRDP
+CGCONTRDP: 3,6,vzwinternet,100.103.33.157,38.0.16.18.177.82.161.115.0.0.0.21.203.11.145.1,0.255.101.0 254.128.0.0.0.0.0.0.0.0.0.21.203.11.145.64,198.224.171.135 32.1.72.136.0.101.255.0.6.46.0.13.0.0.0.0,198.224.169.135 32.1.72.136.0.100.255.0.6.32.0.13.0.0.0.0
OK