Port iptable
#If both returns 1 it's ok. If not do the following:
sysctl net.ipv4.conf.eth0.forwarding=1
#or
echo '1' | sudo tee /proc/sys/net/ipv4/conf/ppp0/forwarding
echo '1' | sudo tee /proc/sys/net/ipv4/conf/eth0/forwarding
#Second thing - DNAT could be applied on nat table only. So, your rule should be extended by adding table specification as well (-t nat):
iptables -t nat -A PREROUTING -p tcp -i ppp0 --dport 8001 -j DNAT --to-destination 192.168.1.200:8080
iptables -A FORWARD -p tcp -d 192.168.1.200 --dport 8080 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
#Both rules are applied only to TCP traffic (if you want to alter UDP as well, you need to provide similar rules but with -p udp option set).
#Last, but not least is routing configuration. Type:
ip route
#and check if 192.168.1.0/24 is among returned routing entries.
DreamCoder