@MMMcQ BE9300 does negotiate 2.5 G on WAN (internet) port for me. There is nowhere to find it in the router control web page, but you can force port speed instead of auto-negotiate in Network> Internet page. Then you may be able to see leds to change on the ISP modem side if it connects and has leds. I don't know what color it should be for 2.5G, it varies between devices.
At least one port is 2.5G as well on the LAN side but I didn't tested it yet.

You may start with a short premade patch cable next to the cable modem, not DIY one. If it doesn't connect at 2.5G I would suspect ISP modem issue.

  @Rch11 

Hey Rich thanks for the reply. I've set the negotiation on the router to 2.5 and auto negotiate and the ISP stated they're seeing the 2.5 negotiating on their side as well. If I power cycle the router I'll get duplex lights for a bit then it kicks back to half duplex. The green light on top of the RJ45 + the amber on the bottom denotes 2.5. 

Then in a minute it kicks back to half with just one amber light on the bottom.  I've tried all different patch cables, both homemade and store bought to no avail and tested each with the mid level Klein network cable tester.  I thought maybe I had a short downstream so I've isolated the router in other tests, no good. I can also negotiate other duplex modes with other devices. I have 2.5 devices showing up tomorrow to ensure I'm targeting only 2.5. 
 

Does your modem show the two led indicators (2.5 duplex) on the modem with no other 2.5 devices downstream of the router? 

I'm not sure we are on the same page regarding speed and half/full duplex. There are few options:

2.5G full duplex.

2.5G half duplex.

1G full duplex.

1G half duplex.

etc.

Half duplex is basically rare phenomena of broken cables. I don't have the same modem and I am not familiar with devices that indicate duplex status on ethernet ports. Only speeds.

Just recently I was pointed that tp-link router actually shows negotiated ethernet port speeds on their web control panel but in a different place than wi-fi clients. You need to go to Network map and click on "Archer BE9300". It shows "Internet 2.5Gbit full duplex for me"

If yours is 1Gbps, I would try to isolate the problem by connecting a third 2.5G device (if you have it) to router and modem separately on the same ports.

  @Rch11 

Rch11, since you are the only one that replied so far, I'll make sure you get a response back. I do appreciate you getting back to me.

So, I've found the solution...not necessarily by chance but by hope.Only on hope because most places say no, do not put a switch between your modem and your router. However, this was the fix.

I bought a managed switch (the cheapest I could find for proof of concept purposes) and plugged her in. All of my devices that are capable of 2.5 G are now 2.5 G.  I have my green and amber lights on the modem ethernet port and average speeds from my direct wired PC to the switch are ~1200 mbps. Now, this leaves me with the gripe that someone, somewhere, and I'm thinking right here at TPLink has some quesions to answer.  What is the difference between the managed switch and the router? When I plugged my PC directly into the cable modem with my 2.5G dongle, it worked like a charm. I tested this before I set up the switch. So I'm thoroughly convinced TPLink has an issue here that they aren't divulging to their customers....hopefully only due to ignorance.  I can't be the only one with this issue.

Unfortunatelyt though, the ISP seems to be taking full control of the switch and I cannot run any of the other ports on to other devices. Ironically I still get internet from my wired PC on the switch, but everything else has to go through the TPLink router in order to function.  So now I have THAT issue I'd like to work out.  Running the desired ports as a VLAN didnt work out.

Router does NAT, a switch doesn't. So you will not get more than 1 device on internet without NAT as ISP provides only 1 IP address.

At this point I lost track what results are you getting connecting the devices different ways. I guess you should contact tp-link directly.

  @Rch11 

Still working with TP link and the ISP in tandem.  It's like two days per step.

So my issue with the NAT idea is that I can parallel my PC and the TPLink off the switch and everything is fine, almost like my ISP allows double NAT. BUT...the management page for my switch is gone when I'm functionally running.  The idea was that I could split ports 1-3 off of the switch to allow it to run the WAN side of things then split off ports 3-8 as internal LAN off of a VLAN but i can't understand why it doesnt like that idea.  Is there something I'm missing above and beyond simply setting up two untagged VLANS?

I just cant for the life of me understand how I can go grab some cheap old imported switch and have it fix this issue but these fancy routers have me hagning out to dry.  I'll try and keep this post updated as things progress

  @MMMcQ  You should figure out if the router negotiates the 2.5G speed at ethernet level at all. This is lower level before any tcp/ip or internet. I had wrote above where to look.

If it doesn't, or does with other devices but has trouble negotiating with your modem port specifically, let TP-link engineers to figure it out or return. I think Walmart (BE9300 is Walmart exclusive version) has 3 month return window.

It makes zero sense to introduce some switch that would just add latency or another point of failure. There are plenty of other router choices.

A switch can't create new local IP addresses (no NAT) so I don't understand what sense you can make out of it between modem and router, other than a fancy cable coupler.

  @Rch11 

Rch,

I wish I could answer your question about what the switch is right now other than a fancy cable connector. It's clearly doing something, but what, I havent gotten into. We are certainly negotiated at the proper 2.5 data link. This is made obvious by the settings verified on the routers network page, the green and amber LED's on the modems RJ45 port, the ports on the network switch, the 2.5 nic light and link speed on my NUC, and the speedtest.net tests.  There is no question about the proper data link being present.

I hesitated to switch away from TPLink as I've been very comfortable with them over the years but this is majorly frustrating. Your advice is advice I may have to take. Of course I've tried to file a warranty case on the AX6000 and they want me to do five more weeks of testing in order to follow their checklist.  It's hard to believe this issue is just isolated to me and there arent any clear remedies out there. I just happened to find this "swtich jumper trick" in some deep dark corner of a forum (not TPLink comminty) dating back to 2022.

Should I be able to get the switch to function the way I want it to (WAN on ports 1,2,3 and LAN on ports 4,5,6,7,8) I will have no qualms being patient for TPLink to come up with some answers.  I realize it's sort of separate from the TPLink issue, but this is all part and parcel of the TPLink foundational issue.