Wireless access in shed

[king1]

so you need a wifi Access Point - something like this will do
https://www.pbtech.co.nz/product/NAPTPL1...0-Multiple

But it will have no ethernet ports spare just wifi - use a switch first if you do

You can use some older modems/routers if they specifically have a WAN connection option, but if they are DSL only modems it might not... I have heard stories that people just plug them in and it just works, they don't for me when I have tried...

as a router (ie via a wan port) it would normally be an isolated network on its own, isolated from the house, so no network shares or shared printers. You can try disabling the dhcp server in the secondary router and setting
its own IP to an address outside the primary DHCP server range, but it gets messy and this is where it breaks down whenever I've tried it, so I usually just get the correct device ...