After reading only a bit about uci I logged in via ssh an set the pdp type:
uci set network.ppp.pdptype='ipv4v6'
uci commit
/etc/init.d/network restart
I got an IPv6 address but the configuration was owerwritten at the next boot. Next I'll try figure out how to make the setting permanent, maybe someone here already knows?
FW ver.: RUT2XX_R_00.01.14.3