1. what ever you read where ever you read you are missing this line “modprobe sit” so run it.
2. once you get your IPv6 from tunnel broker you need following.

NOTE : change "Internal_ip" "Tunnel_broker_ip" "IPv6" "IPv6_gateway_ip" with information you get from your broker

| ifcfg-sit1

STARTMODE='onboot'
BOOTPROTO='static'
TUNNEL='sit'
TUNNEL_LOCAL_IPADDR='Internal_ip'
TUNNEL_REMOTE_IPADDR='Tunnel_broker_ip'
IPADDR='IPv6'
TUNNEL_TTL='64'

| ifroute-sit1

default IPv6_gateway_ip

1. Now turn on the ipv6 using ipup sit1

You should be good to go