Alas, as grateful as I am for your help, that information does not solve my problem.
I do not wish to gain remote access to their network from mine. I wish to offer remote access to my network from theirs, and for security reasons restrict the IP addresses that are allowed to attempt to connect to my network.
I am aware of dynamic DNS services, and have an account with no-ip.org.
I thank you for the time and effort you have taken to respond to my query.
I have several solutions available, the simplest would be to lock down access by IP address.
If that means asking my father to request a static IP I may do that.