I'm using the paid service from windscribe with a Pi as a VPN router for my lan. Works fine for me. (Pi3 acting as AP, DNS, DHCP, and router)
I can see no reason that the free service wouldn't work.
Unless they've changed things recently, you'll need to set things up manually as their limux tools have been built for a different CPU type - x86 rather than Arm.
Not sure what steps are needed on libreelec but essentialy you install and configure openVPN.
I do hope you're not intending to use the VPN for nefarious purposes (accessing licensed content without paying for it for example)
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