@CornishNeil 

From some recent testing, please allow me to offer some info on the results.  As a secondary router, an AX55 (V1) was tested as a VPN client using both an OpenVPN and a WireGuard connection through SurfShark.  Both connections that were tested had to use double NAT and both worked OK even though the speeds were nothing to brag about.  The WireGuard connection was the fastest.

The AX55 allows you to configure a VPN client and then allows you to configure which clients will use the connection (such as your Smart TV).  The rest of the devices will use the normal connection.  I'm sure other Tp-Link routers will do the same but many of their online emulators are not updated, making the router choice more difficult if you want to examine their web interfaces.

Also, from experience, you don't need a super fast connection with a Smart TV and a WireGuard connection will work fine.

  @jra11500 Hi thanks for that, very helpful. Is the 'TP Link Archer AX55 AX3000 Wireless Dual Band Router' the one you refer to? Do you have to do any special set-up to the TP Link router to make it work as a slave to the main router or is it just a simple connection via the LAN ports?

  @CornishNeil 

Yes, that's the router.  If you decide to go with it, be advised that there are several versions.  All of my AX55s are Version 1 and the firmware supports VPN servers and clients.

For setup, it's just a matter of connecting a LAN port of your main router to the WAN port of the AX55.

  @jra11500 Ideal, thanks again. I will source a router and give it a go.

  @jra11500 🙏 I planing to do exactly the SAME however NOT just for Smart TV but for my Laptops as well, my main Router is AX73, sadly in India AX20/AX55 kind of out of stock, so maybe  AX72/AX80 will be 2. VPN Router!  

About the speed i would like to play via GeForce NOW so need high speed my Internet is 300Mbs via Windows/NordVPN i get like >200Mps, i guess the speed is depends on how "powerful" the CPU of the Routers, right? I mean bigger Routers are better! 😊

Thanks  

  @Kali777 

I am not a gamer so I don't know what speeds are necessary for games like GeForce NOW.  I just tested this PC with a SurfShark NY connection (from Europe) and the download and upload speeds were around 80-100 Mbps.  Without the VPN, the speeds are around 250-300 Mbps (the PC's Wi-Fi adapter is not the greatest) on my 1 Gbps network.

On other occasions. I remember the VPN speeds being as low as 50 Mbps.

  @jra11500 
Thanks for the update! 100 Mbps should be enough! 

Maybe did You Tested on main Router as well please? Would be interesting to know how much over head coz of Double NAT, I will try it on my AX73 
I have LOT of IoT Devices, plus Smart Speakers/TV Sticks/Laptops/TVs/etc.. i already setup "fixed" >40x DHCP Address Reservation on AX73, so i don't want send every Devices traffic via VPN... 
BTW FYR similar setup with details Need help setting up the openvpn client on secondary router Archer AX55

  @Kali777 

I can not test the VPN connection on the main router because the main router belongs to the ISP and is not VPN configurable.

My current setup consists of two independent LANs connected to the main ISP router.  One LAN is for IoT devices, guests and IPTV services.  The other LAN is a private LAN for PCs, iPhones, etc. The IoT LAN uses an AX55 in wireless router mode with an additional AX55 in access point mode.  The private LAN uses two AX55 routers in an EasyMesh configuration.  With both LANs connected to he ISP router, double NAT is a reality but does not cause any problems whatsoever with the VPN or with devices such as my IP cameras.  In all, I have some 40 devices connected and the AX55s are working smoothly.

Let me comment that I only use the VPN (occasionally) for PCs on the private LAN and Fire TV devices on the IoT LAN.

  @jra11500 
Aha, okay, got it, thanks to clraify so You have different LANs that's why! You "lucky" to have SW LAN Ports on ISP Modem/Router, mine has ONLY 1 LAN Port! 

I have big House my setup is little bit "complex" with 3x Powerline Adapter, 4x Router, 2x SW like this! 

2. Floor INET <=> Modem <=> AX73 Router <=> PA4010 <=> 1. Floor 

2. Floor Balcony C80 Router WDS <=> AX73 Router

1. Floor PA4010 <=> C7 Router <=> WPA4220 <=> C3150 Router LAN <=> TP-Link SW <=> TV/Smarthings/Philips HUE

I have different 3x different Wi-Fi for IoT & TVs/Laptops/Mobiles but i'm using SAME LAN for simplicity!