R6700v3, Cox, and IPv6

With this router (firmware V1.0.5.128_10.0.104 ) connected via a CM500-100NAS modem (firmware HW versio n C312T00-01; firmware V1.01.11), my hosts get a gamut of temporary / dynamic IPv6 addresses.  This make sit kinda tough to configure remote firewalls!  I asked Cox about how to get permanent addresses, and they took the "Oh, you own your modem and router, we can't help you" road. 

It took some fiddling to get even that, so I'm not super eager to start experimenting with different settings.  Does anyone KNOW (not have an idea or a guess) how to configure the v6 settings in this router?

And, yes, I know it's old... I may have to replace.  But with a couple of extenders my wifi is working pretty well, and I'm not super eager to spend a bunch of money and have to fiddle with everything unless I have to!