Using a Static IP Address on a Raspberry Pi Connected to my Nighthawk Router

Hi,

I have a couple of Raspberry Pis connected to my Home Network which is provided by my Netgear Router.  Ideally I want to set up a static IP Address for each of these Pis, but I cannot make them 'see' the Internet reliably unless I use Address Reservation in LAN Setup page.  This works, but it means that I can only have the IP Address that the Router gave me through DHCP before I reserved it.  What I really want is to specify static IP Addresses in the 'dhcpcd.conf' file on the Pis and have the Router accept these as the addresses to use.

I'm sure that I was able to do this in the past, but not any more.

Also.  If I reserve the DHCP Server on the Router gave me, I'm sure I used to be able to edit this to the IP Address that I want but this also causes the system to fail.  (For info.  The Router assigns the address and I can connect to either Pi using SSH via the LAN, but the Pis can't see the Internet.)

Is there a way to do this?