I've heard people had luck with Tailscale playing nicely with non Plex options. I can't say I've tried it. Though I do use Tailscale. Essentially if you setup Tailscale for Grandma it'll be like she's sitting on your local network. Even better, set it up on her router and you can literally debug all her Internet problems if you can ping it.

Beyond that a raspberry pi with a battery backup on a 4G subscription connected to the router. That would be the ultimate "grandma" setup. Connect her router/modem power to remote power cycle. But I digress.