What is the function of the red WAN socket on the back of my BT Homehub 5?

The red socket on the back of the HomeHub5 is to connect the Hub to an Openreach modem and thus provide a network facility for the whole house, wireless or via ethernet cable. Since the Homehuh 5 has the modem built in anyway it seems a bit redundant unless you have an RJ45 (ethernet) plug connecting to your telephone line rather than the more usual similar but slightly smaller RJ11 plug which goes in the grey socket.
By the way I thank you for your testimonial and could find no other way to answer it than answering this former question of yours! Maybe I'll find a better way later on !
Anyway all you need to connect your Netgear is to use an ethernet cable from one of the yellow ports on the HomeHub and then configure the Netgear as necessary to act as a wireless access point, or wired router. The HomeHub5 is however quite a good wireless access point working at both 2.4Ghz and 5GHz bands to give you the best speed, as well as the network ports (yellow) on the back.

The function of any WAN socket on a device is to input signals from outside your house (for example, the internet). Plug an ethernet cable into it and then the other end into a wired router and you will have WiFi throughout your house.