Access Instance Port from Public IP
As a helpful r/oraclecloud user mentioned, Oracle cloud images use iptables, not UFW for ISCSI support. Here are some notes from the conversation:
This is useful for testing to make sure the port works, but is very insecure:
Add Open Port to IPTables
This is more secure. Adds a rule for some given port, in this case port 80:
iptables -I INPUT -p tcp -m tcp --dport 80 -j ACCEPT iptables-save > /etc/iptables/rules.v4
Thanks to Reddit's /u/BarBarian for his help with my woes.
Note: This assumes you have the virtual cloud network's subnet security rules set properly (e.g. port 80 0.0.0.0/0 ingress/egress rule).
Set DNS Servers for Instances
This one also proved rather annoying. Oracle does some funny business with the systemd-resolve that makes it so the normal route of specifying nameservers in /etc/netplan/* doesn't work. Instead, navigate to the Cloud Panel > Instance > Virtual Network > DHCP Options > Edit the active DHCP option set > Custom Nameservers.