@andreamike 

Internet Service Providers that have a strong position in the market always seem to be looking for ways how they can "force" their customers to use the ISP-provided routers (or "hubs", as they often like to call them).

Implementing new standards that aren't supported by home routers that are available on the free market is certainly a clever way to achieve the above.

European law requires ISPs to disclose the details of the protocols they use so that third-party router manufacturers are able to design and sell compatible alternatives, but it might take TP-Link and the other manufacturers some time before they come up with products that support MAP-T.

If you setup the AX73 in router mode with dynamic or static IP and connect its WAN/Internet port to a LAN port on the Sky router and then just use it this way would that satisfy your needs? Or do you have special requirements, like direct access from the Internet?

@woozle thank you for your kind reply. Yes, my AX73 router is setup exactly how you are describing it but I need to use the OpenVPN feature but it seams to not work anymore. 

@andreamike 

Are you able to log into the Sky router's configuration webpage?

If yes, can you check if the Sky router still obtains a public IPv4 address from your ISP and if it also has a port forwarding feature?

@woozle 

Thank you for getting back to me. Yes it gets an IPv4 shared (MAP-T) address starting with 101. Yes, the router has portwarfarding feauture. Thanks!

@andreamike 

If the IPv4 address starts with 101..., then I presume it should be reachable from the Internet.

So if you setup port forwarding on the Sky router to forward UDP port 1194 to the internal IP address that is assigned to the WAN port of the AX73, then in theory you should be able to connect to the VPN server of your AX73 from the Internet.

(UDP and port 1194 are the defaults for OpenVPN. If you have changed them to something else, then of course you need to setup the port forwarding accordingly)

@woozle 

Hello, so I finally managed to setup the port forwarding. Sky is making it very nasty by disabling the feature on the standard browser GUI and make it available only for their mobile App.
 

So, everything is setup but as the IPv4 is a shared address using MAP-T, OpenVPN is not connecting. Is there any other way? Or can I use an IPv6 address on OpenVPN config file?

@andreamike 

I've had a look at some OpenVPN configuration guides I found via Google and apparently it's not enough to modify just some settings in the configuration file that you load into the client. You also have to make changes to the configuration file of the OpenVPN server, which resides inside the router and is not accessible to the user on most commercial routers.

Therefore, if the router manufacturer doesn't provide a way to switch between IPv4 and IPv6 transport protocol for the OpenVPN Server in their configuration GUI, then there is nothing the user can do.