In lesson 4 and lesson 5 you point out how to configure static IP addrs. Personally I find it easier to update cmdline.txt, one place to add static IP info, see example below:
cat /boot/firmware/cmdline.txt
console=serial0,115200 console=tty1 root=PARTUUID=edfc373d-02 rootfstype=ext4 fsck.repair=yes rootwait cfg80211.ieee80211_regdom=GB cgroup_memory=1 cgroup_enable=memory ip=192.168.0.199::192.168.0.1:255.255.255.0:cp0:eth0:off
ip=<IPv4_addr>::<default_gateway>::::<dhcp_condition>
With that, my router picks it up automatically, no need to update the router. Just an observation, your guide is one of the best / most complete I have found online.