I find it intriguing that with all your IT skills of setting up your networks, and with a place in the UK, you still pay for a VPN provider when you could very simply run your own or create an free instance in the cloud.
Ref your original question. Have you ever tried travel routers? They are cheap and extremely simple to setup, ideal for VPNs and have settings for working as repeaters, tethering etc. I have a GL.iNet M300N Mango router connected to my Livebox with no issues, and it is also powered directly off the USB port. The signal from the travel router around the house is stronger the that of the Livebox even though it is only 5cm x 5cm. The travel router has my VPN wifi signal and the Livebox the “French” wifi.