Solved using this procedure:
1-Leave DNS forwarder enabled.
2-Place DNS in Services->DHCP server for OPT1. This allows correct host name resolving
3-NOT allow DHCP on wan to override DNS set in System General Setup.
4-Add a rule to allow acces from OPT1 to 15.0.0.1 (OPT1 interface webconfigurator) with Gateway set to default instead of Loadbalancer.
Thanks