Model : TL-PA9020PKIT

Hardware Version :

Firmware Version : 3.2.4

ISP : Spectrum

So I want to have as much of my stuff as possible in my cottage, so I'm trying to do this:
Modem
TP-Link 1 (in main house)
...
TP-Link 2 (in cottage)
Router

When I connect Modem and TP-Links (secured!) as above, and plug my Mac into TP-Link 2 via Ethernet, I can get on Internet no problem--my Modem gives me its IP address, I get everything fine over DHCP, and can get on Internet no problem. (I'm writing this post using that configuration.) But when I plug a router into it instead, the router doesn't see the Modem. I've tried 2 completely different brands, a Netgear, and an Apple Airport Express. I've tried Automatic (DHCP), and I've also tried manually entering the same IP information as was given to my Mac. I've tried resetting the Modem.

I don't understand how I can so easily be getting connected via Ethernet on my Mac, but my routers won't do what should be the same thing. Anyone used this configuration and got it to work? Any tips?