The modem is connected to LAN4/WAN port and iMAC is connected to one of LAN ports, right? Normally, if you configure it as a wireless router and there is WAN IP address, it is done. What's the...

On its web interface, go to Access Management---UPnP and you can activate it. I didn't find AP isolation option on web interface of W8961ND. On Interface Setup---Wireless Page, change channel to 6...

Change MTU to1400. Any other wireless clients work OK by the router? Update the phone to latest version.

David, I don't think TP-LINK wireless routers has such a function to seperate wireless connection from wired connection in different gateways. Not sure if third-party firmware can do it.

Cannot control over 100Mbps since it is the maximum speed given by your ISP.

You may reset all of them to factory defaults and reconfigure them since it has been messed up. I cannot think up how does it happen but it is abnormal obviously. Good luck.