Just to add to that, it might not be the ethernet cable within the walls that is causing the problem, my guess is that the connectors are at fault. Gigabit cabling requires all 4 wire pairs (8 wires) to be connected end to end. If only 2 wire pairs are used, the max rate will be limited to 100 Mb/s. Add to that any problems with the cable, or possibly problems with those connections, and you're down below 100 Mb/s.
Before you move anything, have a look at the back of the CGN3. The connected port LED should be amber, indicating that its running a gigabit connection with all 4 wire pairs. If its green, you're limited to 100 Mb/s if the wires are installed on the connectors correctly.
To check the status of a cable, end to end, you need a network tester such as the following which can be found at Lowes Canada and probably other hardware or computer stores as well. Having one of these around is always a good idea, especially if you have ethernet cabling running through the walls.
These will tell you if the wires are connected in the correct sequence, or if any of the wires are not connected at all. If the sequence is incorrect, or you have an open circuit, its not difficult to install a new keystone and punch down the wires onto a new keystone. If you look behind the wallplate of the ethernet port that is being used, you should see that all 4 wire pairs are in use. If any of the pairs is not connected, and is simply cut off or pulled back along the wire, that would instantly tell you you need to install the other wire pairs. The older wall plates have screw down connectors, where you simply wrap the wire around the screw and screw it down to make contact. Newer wall plates have snap in keystones where the wires are punched down onto the keystone connector. You will have to physically inspect the wallplate to see what is going on.
Here are the keystones:
And the wallplates for the keystones:
@jimboden I would run a factory reset just as an experiment, and then check the NAT Acceleration setting and run a speed test. After that try adjusting the NAT settings to see if it changes the NAT Acceleration setting.
Yup, amber on the CGN3 indicates a gigabit connection. That is the opposite of the industry standard from what I've seen, where Green indicates a gigabit connection.
I think at the present time, the question is, does the in-wall ethernet fully support gigabit data rates, which is what you need in order to run rates over 100 Mb/s. As VivienM indicated, if you move the router down to the modem and connect the two with a Cat 5e or Cat 6 ethernet cable and end up with the correct rates through the router, that definitely would indicate that there is some problem with the in-wall cable or the connectors. From there, its a matter of determining if the issue is with the cable or its connectors. My bet is the connectors.
Edit; With the modem downstairs, and router upstairs, and with both powered up and running, take a look at the back of the CGN3 to see what colour the connected port LED happens to be: amber - 1 Gb/s, green - 10 / 100 Mb/s.
Rereading thru your previous posts, I see that you do have QOS up and running, so, yep, save the config, run a reset for the 68U, check the NAT Acceleration setting after the reset, which should be in Auto, and then run a speedtest to see what the rates are. If you decide to use some function, run a speedtest, before and after the function is turned on to see if it causes the NAT Acceleration to kick off without warning and slow the throughput rate. Unfortunately, there does not appear to be any list of functions which are incompatible with Broadcom's Cut Through Forwarding (CTF). The CTF is proprietary to Broadcom, poorly documented and poorly understood when it comes to consumer products. So, its a matter of being aware that there might be certain functions which are incompatible with CTF and experimenting, looking for adverse impacts on the data throughput when you enable the router functions. Broadcom chipsets are also used by other manufacturers, so everyone is in the same boat.
ok.... so realized that I hadn't turned Adaptive QoS > QoS > Enable Smart QoS to OFF... as soon as I did that and Applied... boom... 150+Mbs
OK, so, the quickest way to determine if there is a connector problem is to look at the back of the wallplate upstairs and see if all 4 wire pairs appear to be connected. Downstairs, typically the ethernet cables terminate in the structured wiring cabinet, along with the RG-6 and telephone Cat 3 cables. You might have an ethernet connecter that plugs right into the modem, or you might have a keystone, from which you run a jumper cable to the modem. You would need to look carefully at either case and determine if all 4 wire pairs are in use. If all of the wire pairs appear to be connected, you will need a network tester as I indicated previously to see which pins are not connected properly to the wires. It is very easy to end up with a pin that doesn't connect or with a wire that isn't in the correct place when connecting the cable to an RJ-45 jack. That is why its easier to install a keystone, and run a short jumper cable from the keystone to a modem, router, switch, etc.
Edit: with the QOS turned off and speeds back up to normal, is that with the router parked upstairs again?
yes... router connected through wall to gateway downstairs... so looks like it was the QoS setting that was responsible all the time...