Mod the limitation away.
It makes absolutely no sense anyhow.

What's special about ore?

You can transport the resources needed to construct the forge & furnace through the portal, but you can't transport the ore?!?

Moreover you can completely bypass the limitation by loading/unloading the ore via a temporary world.

It's a poorly thought out limitation that has no basis in the lore, and exists only to artificially extend the (already substantial) grind.