Issue in assigning fixed DHCP address on Cisco-IOS router, using Ubuntu DHCP server

[ps12]

I have a DHCP Server which is an Ubuntu machine. I am trying to configure cisco IOS routers using dhcp. The routers are configured to receive fixed-DHCP IPs, however, dynamic DHCP configuration works but the fixed-DHCP it is not working.

Ubuntu [DHCP Server]: 16.04.5 LTS
Cisco Router [DHCP Client]: 7200, version 15.2(4)S5

DHCP Server config:

automation@automation:~$ sudo cat /etc/default/isc-dhcp-server | grep INTERFACES
INTERFACES="tap0"

automation@automation:~$ sudo cat /etc/dhcp/dhcpd.conf

ddns-update-style none;
default-lease-time 600;
max-lease-time 7200;

authoritative;

log-facility local7;

subnet 198.51.100.0 netmask 255.255.255.0 {
# range 198.51.100.6 198.51.100.254;
option subnet-mask 255.255.255.0;
option routers 198.51.100.2;

host R1 {
hardware ethernet CA:01:3B:D4:00:08;
fixed-address 198.100.51.4;
}
}

automation@automation:~$ sudo systemctl status isc-dhcp-server
● isc-dhcp-server.service - ISC DHCP IPv4 server
Loaded: loaded (/lib/systemd/system/isc-dhcp-server.service; enabled; vendor 
Active: active (running) since Sun 2019-03-31 13:19:23 MDT; 6min ago
Docs: man:dhcpd(8)
Main PID: 7448 (dhcpd)
CGroup: /system.slice/isc-dhcp-server.service
└─7448 dhcpd -user dhcpd -group dhcpd -f -4 -pf /run/dhcp-server/dhcp

Mar 31 13:24:30 netman dhcpd[7448]: DHCPDISCOVER from ca:01:3b:d4:00:08 via tap0
Mar 31 13:24:59 netman dhcpd[7448]: DHCPDISCOVER from ca:01:3b:d4:00:08 via tap0
Mar 31 13:25:03 netman dhcpd[7448]: DHCPDISCOVER from ca:01:3b:d4:00:08 via tap0

DHCP Client config:

R1#sh int fa0/0
FastEthernet0/0 is up, line protocol is up 
Hardware is i82543 (Livengood), address is ca01.3bd4.0008 (bia ca01.3bd4.0008)
Internet address will be negotiated using DHCP

[balaji.bandi]

Your DHCP Range and allocation IP not matching ?

subnet 198.51.100.0 netmask 255.255.255.0 {
# range 198.51.100.6 198.51.100.254;
option subnet-mask 255.255.255.0;
option routers 198.51.100.2;

host R1 {
hardware ethernet CA:01:3B:D4:00:08;
fixed-address 198.100.51.4;   <<-- this should be in this range right ? (198.51.100.X )
}
}

 Change accordingly and restart DHCP deamon and try again and advise.