Sorry, that makes absolutely no sense at all
Sorry, but I have to agree with @licquorice , it makes no sense at all.
Don't concern yourself with anything on the WAN side of the TP Link, you only need to configure the Local Network side.
I have found a manual at https://static.tp-link.com/2018/201804/20180412/1910012403_TD-W9960(EU)_UG.pdf , hopefully it will be of use even if there are slight differences.
The Gateway address can clearly be seen in the picture on page 55 Para 13.1.2.(2)
I repeat, a static IP address is preferable to a reserved DHCP address.
Don't take this the wrong way, but I fear you don't have a complete understanding of how this all works.
The IP address of the TP Link is irrelevant in reality as long as it is in the same subnet as the hub, i.e it is 192.168.1.XXX.
It is only needed as a means of access to the management page of the TP Link. We recommend using an address outside of the DHCP range of the hub to avoid any problems.
When properly configured, you will have one network with 4 different wifi SSIDs, a 2.4Ghz and 5Ghz from each of the devices. As an aside make sure you don't use the same frequencies on both devices as it may cause interference. The MAC connects to whichever device by wifi name, not by IP address.
Try this generic guide or check Youtube for a video.
https://www.cnet.com/how-to/how-to-turn-an-old-wi-fi-router-into-an-access-point/
My Mac would not connect to the TPLink wireless and therefore I could not connect to the console on an IP specified outside the range of the HH4 e.g. 192.168.1.1. I needed to use the assigned IP from the HH4 for the TPLink console IP. I could then access it for administration from my Mac once I had connected to my HH4 wireless network. Anyway, I have remedied the Mac connection issue by reseting the network connection on the Mac and restarting the HH4.
Again, many thanks for your help. For now, all seems working.
That really doesn't make sense, but glad you got it working. You don't have to connect to the TP Link wireless network in order to connect to the TP link management page regardless of its IP address as long as you are connecting LAN to LAN and everything is in the same subnet.