To add to my grief, I was wrong about the distance between the routers, doubling the distance. 🤦♂️ It was actually a 25ft cable and I needed a 30ft cable to make it one connection. Let's just say...

It's a "business" router... for getting "business" to coupler and wire dealers. :P :P :P

:P

Still an issue. Going with the 60-foot cable. What a freaking hassle TP-Link!

Unfortunately this seems to still be a problem. It appears that if I actively use the ER605 unit (e.g. continuously mashing the Refresh on Port Status), the port stays at 1000. But if I stop using it...

What a hassle. Looks like those couplers were the problem. Confoundingly complicated by the fact that they worked without the use of ER605. Moving to these has gotten us 1000M connection for 4 days...