Router most likely will have a subnet mask 192.168.1.1 (or other private address) > 255.255.255.0

Like yourself ive turned off DHCP so this might help; a example of my setup :-
Router IP = 192.168.1.1 / 192.168.1.0 / 255.255.255.0


click connectivity settings > connections > yourwifi > edit

connection name = anything
Connection type = WLAN (most likely for home wifi)
Network name SSID = set via router
Network hidden = yes/no set via router
Netowrk mode = infrastucture
Security method = WPA pre shared or set via router
Pre-shared key = xxxxxxxxxx

Advanced
IP Address = 192.168.1.94 (you pick within router range)
Subnet mask = 255.255.255.0
Default gateway = 192.168.1.1
Primary DNS address = 192.168.1.1
Secondary DNS address = 192.168.1.1