An OpenVPN solution could, in theory, tackle your usecase! I have considered trying it multiple times but I have never gotten around to try it myself (no real use case for me, other than just for proving it can be done).
But I have gained a lot of experience with OpenVPN in the past, which makes me think it could be possible. OpenVPN can use the tunnel against 2 types of devices: "tun" and "tap".
The regular mode of usage is "tun", in which the OpenVPN tunnel will have an IP address at both sides of the tunnel and network segments at both sides will use the 'remote' tunnel endpoint as their gateway if a network segment at the other side of the tunnel is to be accessed. This would be, simply said, the use case for a site to site VPN.
But the "tap" device, available in many (if not most) Linux kernels can do some 'magic'. When you set up an OpenVPN tunnel in "tap" mode and you tie the "tap" device as well as a regular "ethX" device together in a linux 'bridge', you are in fact creating sort of a 'switch'.
So you take 2 Linux boxes (can very well be dd-wrt or OpenWRT, although I do not know if they have the tap module in their kernel). You take all router functionality out (or disable it) and you just use 2 LAN interfaces. One LAN interface is used to set up the secured tunnel (so it needs access to your regular home router). The other LAN interface is connected to the KPN IPTV network (either directly in the designated port in the Arcadyan or a simple switch that is connected to that port).
The LAN interface that is connected to the KPN IPTV network has to be put in to a Linux bridge device. You do the same with the TAP device used by OpenVPN. In your other house, you do the same. But this time, you can connect the Motorola set top box to the LAN port that is in the 'IPTV bridge' at that side.
Aside from OpenVPN, modern Linux distributions and kernels contain support for "l2tp" (layer 2 tunneling nprotocol). Layer 2 is referring to the layer in the OSI networking model on top of which IP (so also IPTV) and other networking protocols like also DHCP, etc. is used.
Apologies for my long (and probably quite abstract) story. But it is complex matter. Even if I had already tested it in practice, I would probably have done that using 2 regular Linux boxes. Handling of LAN interfaces on embedded (router) devices comes in all kinds of flavors.
Marstek Venus 5.12kWh v151, CT002 V118, CT003 V116 DSMR5.5, PV 11xEnphase IQ7+ Z-O, 5xEnphase IQ7+ N-W - ~4,7Wp theoretisch, ~3,5Wp praktijk.