Ethernet - Router not assigning IP addresses to some devices.

I want to know then reason for your problem as a matter of curiosity. Things should not be happening the way you describe, yet I have no doubt they are. So following Mr. Spock's advise I'm eliminating all logical reasons to find the illogical. 

Absolutely!

I’m so very confused on what’s going on as well, and now dedicated to fixing it. 😂

Im not in IT, but I am a software developer, so I work on these machines all the time. This is so random and I’ve troubleshooted all the things I could think of. 

The intermittent connection in windows make me wonder if a lower throughput would have an effect. 

Right click Start and select Device Manager. You have both wired and wireless network adaptors the wireless usually are identified as such. Right click on the wired adaptor and select Properties. Under the Advanced tab there should be a Speed & Duplex setting. It's probably on Auto Negotiate select 100Mbps full duplex instead.

@jenniferrr   So I think you confirmed that the two computers are in the same room and connected to the same switch, right?  Are there any other wired devices connected to that switch and, if so, do they work ok?

This still seems to be a DHCP issue and I've seen situations where switches are having issues communicating with the router/gateway and they will give out one IP but when another machine shows up and requests an IP, the switch tries to give it the same IP.

So perhaps the cable between that switch and the wall has an issue, or the switch or even one port on the switch itself does?

Very good ideas here - @lesrof2evils  I cannot try the Speed & Duplex setting until later this evening, but will give it a go. 

@tonydi this is correct, both computers are in the office connected to the switch. What is strange though is the errors are still persisting on the iMac regardless if it’s the plugged into the switch or directly to the wall jack. With the Windows machine, it will connect via the wall jack sometimes, but it’s intermittent. I’ll have to check again later after work to see if it will connect. 

Ahh, ok, then since that wall jack connects with another switch upstream on the way to the BGW, it could be that you'll need to look there.  Your whole setup is like one of those domino chains, lots of places where if one thing tips over it can mess up everything. 😁

Ok, sorry it took me a while to get back to this! 

I have changed out the switches (office and main), and all the cables connecting the two. 

Issue is still happening **SIGH** 

You talked about the iMac being too difficult to move there and the Win machine connecting directly to a wall port.  One thing that I don't think you ever did was to connect the Windows computer directly to the BGW.  That would certainly eliminate everything except the computer and the gateway.

Just to be sure you're back to square one, both computers are set to obtain an IP automatically, right?  No forced static IP on either?

Grab something like the Fing mobile app and run it to get a listing of all of the devices on the network.  Compare that to what the two computers think their IP's should be. 

Ok, I connected the Windows machine to the BGW and it connected right away. 

Everything is set back to square one on both computers - no forced static IP. 

I'll download Fing and see what it says. 

Quick question.....are both switches the exact same model, the TP-Link TL-SG108 (no letter after the 108?)?