DPC3825 firmware update

In 2012 I bought the Cisco dpc3825 that I used to rent from Cox. It has old firmware & needs yo be updated. I contacted your tier 2 support to try to upgrade, but they said their "firmware version update check" failed & so did the upgrade attempt. The cable modem is working correctly (except the DMZ mode), but I changed the login credentials of the dpc3825, so this is why I assume they couldn't do the version check (not sure if you use SNMP, etc for upgrades). Below are the device specs/info.Ideally, I need to put the dpc3825 in bridged mode since I need to forward GRE protocol 47, which of course is neither TCP nor UDP .I'm hoping you can help me?

Model:     Cisco DPC3825
Vendor:     Cisco
Hardware Revision:     1.0
MAC Address:     bc:c8:10:06:8d:80
Bootloader Revision:     2.3.0_R3
Current Software Revision:     dpc3825-v302r125574-170105a-COX
Firmware Name:     dpc3825-v302r125574-170105a-COX.bin
Firmware Build Time:     Jan 5 09:25:59 2017
Cable Modem Status:     Operational
Wireless Network:    Disable