@SpoilerN9 essentially if you want to use tghe digital Sky Talk voice line a Sky hub has to be directly connected to the incoming line. Sky use a proprietary solution for the voice line which should not be confused with third party VOIP systems. Sky hubs do not offer a bridge or "modem" mode so cannot pass the external IP,address to a second router so you will getva double NAT.
In practice Double NAT systems are nowhere near as bad as you may have read and for most people work with no changes beyond turning off the Sky hub's WiFi. The linked thread is pretty unusual and I would suggest initially sticking with the hub's default settings. If you need to do more complex actions like forwading firewall ports then use the Sky hub's DMZ function.
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65inch Sky Glass, 3 Sky Streaming Pucks, Sky Ultrafast + and Sky SR213(white Wifi Max hub) main Wifi from 3 TP-Link Deco M4 units in access point mode
