Spectrum Internet on Ethernet with Two Macs?

I have Spectrum internet service with a Spectrum modem. Modem model is ubee TWC (Time Warner Cable?) DVW32C1. It has four Ethernet ports. Port 1 is connected to a Netgear Nighthawk WIFI master. Port 2 is connected to a 13” Macbook Pro early 2015 running OS Catalina 10.15.7 via a Thunderbolt Ethernet connection. Port 3 is connected to a 21” iMac late 2015 running OS Catalina 10.15.7 via standard Ethernet connection.

Both Macs connect to the internet using WIFI. But if I try to connect both Macs with Ethernet, only one of them is able to connect. The other Mac has a self-assigned IP address and cannot connect to the internet using (DHCP).

Originally, the iMac was able to connect to the internet via Ethernet and the laptop wasn’t. I recycled (powered on and off) the Spectrum modem with both Macs shut down. The power was off the modem for at least 45 minutes. After the modem was re powered up, I started the laptop first. The internet via Thunderbolt internet now works on the laptop. But the iMac cannot access the internet via Ethernet.

I tried using DHCP with manual address on the non-working Mac. It gives me a green light and says the Mac is connected with the address, but the internet won’t work.

I haven’t tried this but I think if I do another recycle on the modem and start the iMac first, the problem will move back to the Macbook Pro.

Is there some reason I cannot have two Macs connected to the internet via Ethernet? It seems like the modem memorizes the last Ethernet internet Mac connection, and no matter what I do I cannot get both Macs connected to the internet via Ethernet. I can change them by recycling modem power.