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.
Something like reverse dynamic DNS for end users? Hm, only if it would be easy to setup, is on the same level as a VPN client I’d say.