Netgear DGN3500: Netgear LB2120 UPnP Unavailable

I'm a bit confused about what you mean by "this device is linked to an actual router".

From the specifications, the LB2120 takes a wired WAN (e.g. from a DSL or cable modem), and also supports 4G cellphone based connections.

Such a setup is fine for fault tolerance or speed boosts, if the 4G network is significantly faster than your DSL or cable connection.

However: most internet providers don't have enough IPv4 adresses for cellphone connections, and therefore use CG-NAT and tunneling techniques like DS-Lite to tunnel IPv4 through an IPv6 address. So multiplayer via cellphone connections hardly work properly.

As for your Nexxt Solutions router: If that really is a router, and not just a cable modem, then you have a double NAT, as the Nexxt router and your LB2120 are both NAT devices.

And, to top it off, your ISP might also do CG-NAT on your cable/DSL connection, which would give you a triple NAT.

Is there a specific reason to why you need the LB2012 for your Xbox? If it's about the "speed" you'd get from the 4G connection, you can use that speed only for downloading games faster. It will not help you for multiplayer or Party chat connections.

If possible, connect your Xbox as "close" as possible to the internet connection with as few devices between the console and internet. Which means to plug it into the Nexxt Solutions router.

When on the router, use your PC to sign into it's configuration page, and take a note of the Nexxt Solutions WAN IP address (i.e. the IP you get from your internet provider). Then surf to a page like https://www.whatismyip.com/ and
make a note of the "Your Public IP" address. If the public IP from the page and the WAN IP on your Nexxt Solutions router are different, your ISP is using CG-NAT and you need to contact him to ask for a proper public IPv4 Address that is not NATed.

there is UPnP on this device. Select Advance from the top, then Advanced on the left, then in the middle of the page you will see "Universal Plug and Play" after selecting this you will have the option to enable and disable.

Having a number of multiple xbox one NAT issues (particularly with Destiny) with this gateway.

what exactly does this do?

I have a netgear CG3000D-RG. UpnP is on.