Hello @Gleon69cr
Any time I see an Ethernet connection running at just under 100Mbps, I have to first suspect the cable or port. There are 4 pairs of wires (8 wires total), and if even one of those 8 wires doesn't make a solid connection, the connection will fall back to 100Mbps instead of 1Gbps.
You've tried multiple cables, but it could still be the issue. If they are stiff cat6 or better cables, they can end up not making good contact due to a tight bend required near the port connection. A more flexible cat5e cable may make a better connection.
You've tested a connection directly to the primary Nest WiFi Router unit's LAN Ethernet port, and it showed the expected speed. So, we know that port is good. That is the port your secondary Nest WiFi Router unit should be connected to. But, for that secondary Nest WiFi Router unit, you can use either one of its Ethernet ports. You haven't said which port you're using on that unit, but, you could disconnect it from power, and then connect your Ethernet cable to the other port than the one you've been using before powering it back up and testing again.
Lastly, I sometimes recommend moving the secondary unit temporarily into the same room as the primary and testing with a short, known good, cat 5e cable and running a test that way just to isolate where the issue may be prior to moving it back into the position you want it to be.
I hope some of these ideas can help you figure out where the trouble is – lots of people have made this work, so I expect you can as well.
