On my Netgear router it’s very easy…
1) Power up the device for which the address is to be reserved.
2) Log in to router.
3) Choose ADVANCED/SETUP/LAN SETUP.
4) Under Address Reservation choose ADD.
5) Select the address you want to make permanent (make certain it’s the device in (1) above.
6) Log off the router, all done.
Most other routers I have encountered are very similar.