I think the problem is that the firewall rules need to be adjusted slightly:
# iptables -L -v -n -t nat
Chain PREROUTING (policy ACCEPT 57641 packets, 3434K bytes)
pkts bytes target prot opt in out source destination
Chain POSTROUTING (policy ACCEPT 998K packets, 73M bytes)
pkts bytes target prot opt in out source destination
128 10658 MASQUERADE 0 -- * eth0 192.168.80.0/24 !192.168.80.0/24
Chain OUTPUT (policy ACCEPT 998K packets, 73M bytes)
pkts bytes target prot opt in out source destination
In your case I suspect that in the "MASQUERADE" rule "eth0" should be "ppp0" (or similar), but I have no idea where to change that.