This wouldn't increase your download speed at all. That is going to rely on your internet connection speed. This can usually be around 20Mbps, your milage may vary. That is why a 100Mbps WAN port is plenty for most home users. Upgrading to a modem/router with gigabit ethernet would increase the speed of your internal network though if you are streaming video or transferring large files.
Cause i was quite confused as i thought only the router gigabit would provide transfer speed through the network not through the modem.
Optimum ultra gives download speed of 101 mbps so does that mean the i would need to get an 1000mbps WAN to obtain that 1 mbps?
Modems have different downstreams. If the one provided is less than the one that i am paying for would that bottleneck my speed. Say the speed I payed for is 50mbps and the downstream for the modem is like 14mbps.