So far I’ve seen WAN leases expire after a long time, say months, or quarter year, so is doable. If becomes an issue I’ll work with them on a VPN solution but is a pain for non-technical users or non-supported hardware. That’s also why I explain “use from your home network only”.

Ask them to visit https://ipv4.icanhazip.com/ and give you back the number, then whitelist in your webserver, as well as your LAN/VPN range, deny rest. Explain they can only reach jellyfin from their home internet. Repeat if they get 403 forbidden after they get a new WAN IP.

That or VPN like openziti, wireguard but gets more complicated.