because my ISP installed a router box, my router box has RFC1918 IPv4 addresses on both interfaces
The tunnelling protocol used with HE is 6in4, also called protocol 41. 6in4 packets will contain an outer IPv4 header and an inner IPv6 header but no UDP or TCP header.
Your only hope to use 6in4 and HE with the ISP installed a router box is that it has a DMZ feature as described below that you can use.
When an endpoint of a 6in4 tunnel is behind a NAT, one can in some cases still make use of the DMZ feature of a NAT router. The NAT router will then forward all incoming proto-41 packets to the configured host, thus making the tunnel work. Some NAT devices even allow transparent operation of 6in4.