@Mattys60 

Try to change another wired cable or ethernet port to connect and verify whether you can get valid IP address. 

Note: You can change another ethernet port to connect the main Deco to the modem and test the spare one. 

Please click here to check the IP address and gateway and show us a screenshot of the status-details page. 

And you can change another device to test the wired connection. 

Still the same, please hard reset the main Deco or change the slave Deco to the main Deco to do a comparison analysis. 

https://www.tp-link.com/support/faq/2400/ 

And please tell us the model number of the switch.

May it help. 

@Kevin_Z  After more testing. When I take the switch out and just used the 3 deco M5. If I plug into either slaves, it's fine I get an IP No problem. But when I plug into the main. I get nothing. And there is no network status to show you because it just falls back to link local.

What would be my next step?

@Mattys60 

You can get IP address with computer connected to the slave Deco which means that the main Deco works fine, at least one of the ethernet ports works well. 

Have you ever tried to change another ethernet port on the main Deco to connect it to the modem and connect your computer to the second one? Can you get valid IP address then? Or does the main Deco work？

If the main Deco still works, then the two ethernet ports should be working properly. You can change another wired device or cable to give it a go.

While once you changed the LAN port, the main Deco stops working, that means that one of the ethernet port may be faulty. You can contact the retailer or the local tech support to verify whether a replacement is qualified. 

May it help.