Setting up TP-Link TD-W8980 as access point

I'm having issues with getting my TP-Link TD-W8980 to work as an access point. The situation is that I've recently upgraded to fibre broadband, but don't have an openreach modem to connect my ADSL2+ cable modem to. I'm pretty skint right now so don't want to have to buy one if I can help it, so instead I'm looking to use the router the ISP have sent me which will work without an openreach modem (it's a plusnet hub one, basically a BT Homehub 5 in a different colour) as a switch. I want to connect it via LAN to my W8980, then use the W8980 as standard.

I've got the plusnet hub working without issue, and have worked out how to stop it broadcasting wifi so I won't have multiple networks interfering with each other. The problem is I don't know how to set up the W8980 to run through the LAN. I've got the cable plugged into the correct port (4), but I don't know what settings I need. The quick setup page gives me the following options:

Quick setup options picture

I'm not sure if any of these are what I want, or if not then I'm not sure what settings I do want. I think the issue is that I can't find a way of setting the default gateway to the IP address of the plusnet router. I've switched off DHCP on the W8980, but that's about the only thing I know is right.

Apologies if this is a duplicate, I've been looking for a couple of days now and have found bits and pieces of the puzzle but nothing that has sorted it. I've probably rambled on here and not given information you need, so I'll update with anything anyone asks.

Thanks in advance for your help.

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