1. DHCP Reservation.
2. Assign the IP in question on the PC (you didn't write it was not a PC).

As to your claim there is no documentation I'm reading the docs at https://www.netgear.com/support/product/D1500.aspx#docs

What it will NOT give is basic networking skills or training. For this discussion I will take it you know basic IP setup and skills.

OK, I don't see any mention of address reservation in the manual so for this round you have to use the full manual IP setup on your PC (AKA static IP.) For this discussion I take it you are a skilled networker, know to keep out of the IP range for the DHCP IP Pool and pause here.

As to the port forwarding, that is in the documentation so I pause here on that topic.