A small computer, large capacity ssd and two WiFi interfaces (2x usb dongles, or dongle plus usb).

Small computer could be anything: raspberry pi (or generic and), nuc mini pc or laptop. If you want to use it without a plug you’ll need to add a battery, usb c powered devices could be more convent to power from a battery.

A ssd is better for this use case. Not because it’s faster, but they are more resilient to being knocked about and dropped. They are also much smaller, especially M.2, and aren’t fussy about how they are mounted.

The two WiFi interfaces would allow you to create a WiFi bridge to access the internet through a WiFi network and access your media server. It would need some configuration, you may also need to have the computer act as a router if you want to use multiple devices without reconfiguring.

It may be easier to have your device act as a WiFi hotspot and have the media centre automatically connect to it. This would make it difficult for multiple devices to use it simultaneously, and you could accidentally allow the media centre to do all its updating and downloading over your mobile connection.

This type of thing is going to be expensive and troublesome to configure unless your already experienced with that sort of thing.

I think a better solution, especially if you already have a media server. Is to set your media server for external access.

To get media when you don’t have internet, buy a large capacity flash drive (or external ssd/hdd). When you have access to your media server download all the content you want on to the drive. I think iOS jellyfin can do this without much modification.

Once out of range of your media server. Delete the content you’ve watched on your device (iPad) to free up space. Connect the external drive through the usb port on the iPad, copy over the next lot of content you want to watch. Disconnect and then watch the content.

Jellyfin can download the content, but you may need another app to play it when you don’t have access to the media server.

This approach lets multiple people access a much larger amount of media, effectively simultaneously. It doesn’t require a large amount of often expensive local device storage - you use cheap external storage. It much less expensive if it breaks or gets lost and has very little configuration -if you already have a media server running jellyfin.