That's correct. There will be no change to the ethernet. That will still remain in place on the router's WAN port. The coax provides RF for the set-top box's video signal. The router's coax provides MoCA for the set-top box to get Guide and On Demand.
All you need to do is run coax from ONT to a splitter. Then one feed goes to the set-top box and the other feed goes to the router.