IPv6 on a Cisco 877w doesn’t work… well it does… providing you don’t want to use the wireless bit.
rtr1(config)#int dot11radio0
rtr1(config-if)#ipv6 ?
% Unrecognized command
rtr1(config-if)#
Despite what you might read on the interwebs, you can’t actually apply an IPv6 address to either the dot11radio interface or sub interfaces.