In theory as long as the PLC has access to the internet or more importantly the internet has access to the PLC you can talk to it.
You might need to set up some port forwarding rules in your firewall to allow access from the internet to the plc but it’s possible. Make sure what ever you do it’s secure. If you know your remote IP for example that hosts ignition you can only let that one use the port forward rule.
I’m not sure what port ignition uses to talk to the plc but normally Siemens use 102 for the PLC’s which I think differs from the HMI’s from memory.
Are the external IP’s fixed? If that changes things will stop working. Could you set up a VPN connection ?
Another option which is probably easier is Ignition edge. That will give you some store and forward storage also.