Setting up failover is fairly straightforward. There are several guides and howtos on the Internet. I used the following:

ISC: A Basic Guide to Configuring DHCP Failover, and

Steven Diver: ISC DHCP Failover Configuration

Here are the important sections of the respective configuration files. The Main Router has the IP address 192.168.19.1 and the Backup Router has 192.168.19.44.

# dhcpd.conf
#
# Configuration file for ISC dhcpd
#
# Configuration of failover
#
failover peer "failover-partner" {
   primary;
   address 192.168.19.1;
   peer address 192.168.19.44;
   max-response-delay 60;
   max-unacked-updates 10;
   mclt 3600;
   split 128;
   load balance max seconds 3;
}
.....
# Subnet specification for unknown clients, primarily on the WLAN
subnet 192.168.19.0 netmask 255.255.255.0 {
  pool {
    failover peer "failover-partner" ;
    range 192.168.19.241 192.168.19.253;
  }
  # .254 is reserved for the 'floating' IP address of VRRP
}

# Subnet specification for IOT devices on VLAN 2
subnet 172.19.2.0 netmask 255.255.255.0 {
  pool {
    failover peer "failover-partner" ;
    range 172.19.2.10 172.19.2.99;
    option domain-name-servers 1.1.1.1;
    option ntp-servers 0.dk.pool.ntp.org;
    option ntp-servers 1.dk.pool.ntp.org;
  }
}
......

The configuration of the Backup Router is almost identical to the Main Router, the difference being the specification of the partner and it not having the line 'split 128'.

# dhcpd.conf
#
# Configuration file for ISC dhcpd
#
# Configuration of failover
#
failover peer "failover-partner" {
   secondary;
   address 192.168.19.44;
   peer address 192.168.19.1;
   max-response-delay 60;
   max-unacked-updates 10;
   load balance max seconds 3;
 }
 ......