Thank you for your continued assistance  @dsj.  I see the following:

https://www.tp-link.com/us/configuration-guides/configuring_network/?configurationId=18572#configuring_port_config_6_4

 

I have applied manual override to the port, we'll see how it does.  I also see Flow Control which seems to be a toggle.  I wonder if that might be a part of this as well.

 

In any case, note that I was able to get nearly two full days of uninterrupted 1000M when bypassing the ER605 altogether via the existing cables/coupler.  I do not think the coupler is a factor here (but who knows!).

 

I will continue to update here accordingly.

Funny you mention that @Virgo because both routers are TP-Link routers. :)  To be sure here the problem is not modem and router, but router to router.  If I plug the modem directly into the 2nd TP-Link (non-ER605) router it maintains 1000M connectivity.  It's only when the ER605 is involved does degradation occur.

 

Unfortunately setting the ER605 port to forced 1000M status did not work.  Trying the flow control now.

Alright @Virgo + @dsj looks like that Flow Control setting is the winner here.

 

 

After setting last yesterday it has been over 100Mb/s on our connection here through the ER605.  It usually takes a couple of hours (or less) to show the degradation so unless something inexplicably occurs after posting this, I am thinking this is the answer here.  I might try setting that Negotiation Mode back to Auto but I am sort of in an "if it ain't broke don't fix it" sort of mood ATM. :P

 

Thank you for your assistance out there. 👍

JUST KIDDING. 😆 It was only waiting another day to degrade to 100Mb/s. So this seems to occur even if Flow Control is enabled and the port is forced to 1000Mb/s. Any further suggestions would be appreciated.

Alright, so I noticed that the cord in the back of the non-ER605 was a little loose, and I was able to further secure it into the socket.  Not sure if that had anything to do with this but it's been 1000M since and it's getting to the point where I feel like I should brag about it here and tempt fate for it to degrade again. 😁

 

Additionally, I did set up some VLANs as it was not properly representative of our intended workgroups.  Interestingly enough, the non-ER605 router was not a fan of the Tagged traffic and had to use Nontag. despite TP-Link's directions:

 

> Untag: The egress rule of the packets transmitted by the port is untagged. If the device connected to the port is an end device, like a PC or a server, the port type should be UNTAG, because end devices don’t recognize tagged packets.

 

Ironically, I was able to restore network connection by connecting directly with a tablet (i.e. a PC) and setting the port from TAG to UNTAG.  Based on the instructions above I shouldn't have been able to do that. 🤷‍♂️

Nope no dice.  However, it seems like degradation is taking longer to occur now.  Whereas it would occur within an hour or two, it now seems to be happening over 24 hours of consistent 1000M connection.

 

Not a lot of options left so I am going back to @dsj and those couplers.  I did try another one I already had but it was the same brand as the one I was using. 

 

So I went with these and just got/installed one:

https://smile.amazon.com/dp/B09FFZ9RHM

 

We'll see how these do. 🤞

 

If that doesn't work I'll look into getting a new single cat5/6 cable and see if that does the trick.  Getting desperate here. :P

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 now. 

https://smile.amazon.com/dp/B09FFZ9RHM

 

I am prepared to mark this closed.  Again. :P

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 altogether the port degrades after about an hour or so.

 

So the reason why it was staying at 1000M for the past 3-4 days was because I was constantly mashing the refresh on the port status.  Go figure that one.

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