Initially you need a new node that wants to join the network to be able to find at least one other node on the network. This means that at the time it first joins, you need there to be another node that it can reach, and it needs you to tell it the address of that node. For that node to be reachable across the internet (assuming you don't have a VPN connecting them) this will mean you need that node's external IP address at that particular moment. You will also need that node to be accessible on that address, which may involve opening firewall ports and/or dealing with NAT settings.
Once two nodes are connected, they should (I think) be able to cope with their IP addresses changing, as long as they don't all change at the same time. This should work because once connected, both nodes have the address of the other node, so if A's address changes, it should still be able to reach B, and let B know about its new address. However you should test this, for example by connecting the nodes then rebooting the ISP's router box on one if them, which will normally get you a new IP address.
However, life is simpler if you can an least provision one node somewhere with a static IP address. That way you can set that address as a boot node for the others, and as long as that stays up, nodes can always reattach themselves reliably regardless of when they stop and start without the risk that you will need to manually tell nodes each others' new IP addresses.
