Hello @Danwatson1988
Any time I see an Ethernet link running at around 100Mbps instead of 1Gbps, I strongly suspect that link is only managing to negotiate and hold a 100Mbps link speed (which uses only one pair of wires) rather than the full 1Gbps link speed (which uses all four pairs of wires in the cable). In your case, I would focus on that very first cable connecting your primary Nest WiFi Pro unit to the first secondary Nest WiFi Pro unit, since everything downstream from there is going through that cable, and everything downstream from that cable is seeing limited speeds.
Keep in mind all of these ports are 1Gbps ports, so they only need Cat 5e cables, which are often more flexible than Cat 6 (or greater). The reduced flexibility of those higher category cables can result in not all pins making good connections in the ports on the Nest WiFi Pro units.
So, I would first just try pulling and reseating both ends of that first cable in the chain between Nest WiFi Pro units. I would also make sure there isn't any side-to-side pressure on the cable end (e.g., pushing the unit up against a wall). If that doesn't improve the performance of the first secondary Nest WiFi Pro unit, then I would try moving it into the same room as the primary Nest WiFi Pro unit (temporarily, just for testing) and then connect it to the primary's LAN Ethernet port using a short, known-good, Cat 5e cable. Verify it shows as wired and that it performs as expected (this may require some creativity, but you can connect a laptop or desktop via Ethernet to the other Ethernet port on that first secondary to run a speed test without using any WiFi).
If that works, you know what to focus on (that first cable run). After that, build up the rest of your system one piece at a time, testing in between each step.
All of that said, I personally prefer putting an inexpensive unmanaged Ethernet switch at the core of the system, connected to the LAN Ethernet port on the primary Nest WiFi Pro unit (not upstream, between that unit and your internet service). Then, run connections independently to each secondary Nest WiFi Pro unit from the switch. Yes, a daisy-chain Ethernet topology can work. But, using a star topology with a switch at the center is going to be a bit more reliable, and it isolates any issues to a single unit, rather than letting that become the weak link in the chain.
I hope these ideas help!
