Can't update N600 (C3700 100NAS) firmware nor disable IPv6

Am unsure whether issue remains unresolved, as original post and comments are somewhat dated, but am sharing my experience that may be of benefit...

Similarly have C3700 unit (originally purchased with OEM firmware V1.01.11) which has been upgraded to current V2.02.08 by Comcast xfinity ISP.

As DarrenM has commented below, Netgear cannot directly upgrade firmware (and does not provide means for customer to upgrade firmware themselves) -- firmware version is controlled entirely by ISP.

That said, for Comcast xfinity ISP, contacting their general technical support line is generally useless, as likely you will be connected to offshore call center whose operators have little to no technical expertise and simply regurgitating call script. Instead, directly contact Comcast Tier 2/3 Advanced Repair Dept in Denver, CO (higher level support that call centers eventually escalate to) -- their direct phone number is: 800-718-7419

True, as has been commented on other related posts, Comcast xfinity ISP cannot manually "force" firmware updates, as firmware updates are automatically controlled by their system infrastructure. Instead, what they can do, specifically have them re-provision your cable modem, which as part of provisioning process of identifying and connecting hardware credentials to your account, it will determine your modem's firmware version and download update if necessary.

Note, as indicated above, as of date, V2.02.08 is current firmware version, however, currently have open Netgear Support ticket (see post: https://community.netgear.com/t5/Cable-Modems-Routers/embedded-web-interface-fails-after-week-runtime/m-p/1329401#M9437) in which Netgear has identified firmware flaw in that current version as root-cause of failing embedded web interface known issue, and is supposed to release firmware fix sometime.