Remove ETH00 from NATted interface, you have to NAT only the ppp, which is your WAN interface (maybe is natted by default, you can check it in Setup, Network, pppX, configure button.
If you have Natted the ETH00 for other reasons, eg for reach your internal servers via FQDN, but from some hosts which are inside the lan, there are other methods for this.
Regards