You can get 2.5Gbps wired backhaul only using the XT8s in AP mode and using a better router as the main node (I use an Asus GT-AX11000 Pro).
See this thread:
https://www.snbforums.com/threads/asus-zenwifi-xt8-2-5gb-backhaul-cat6-link.71314
When you say that are not using the "smart network feature of the AX", do you mean that have disabled also the "802.11ax / WiFi 6 mode" in the wireless 2.4 settings?
If not, try disabling it.
If you are using all the 4 XT8s as access points I suppose you have set 4 different SSIDs....
Or are using 1 of the XT8s as a main node and the other 3 as mesh nodes?
Anyway try resetting the problematic XT8 to factory and set up it again.
As wifi signal seems poor from basement to upstairs due to the building fabric (and this will probably apply to any wifi type connection, even AX), consider also a possible Powerline connection between basement (the main router) and upstairs (a mesh satellite).
In my experience with 3 wired XT8 nodes with backhaul set to "2.5G WAN first" , even if the cable/switch is disconnected, anyway the nodes switch to the wireless backhaul.
In this case the key is "Backhaul Connection Priority" and the word "first"...
Anyway, this method works perfectly in my 5 XT nodes mesh setup. My wifi 6 AX 160 MHz laptops and phones can connect up to 1200 Mbps to the nodes. And if a wifi 6 AX device is wirelessly connected to one of my nodes that can take advantage of the wired backhaul (in my case running at 2.5Gbps)...
I use a non-Asus router/firewall as main router.
In my router the load balance has 2 option that can be configured:
- per session
- per address
If per session is used, no spreading of traffic between the 2 WANs can be done.
If per address is used, speedtest.net using the Multi option actually...
The wifi connection is showed as 1 to 4 bars in the network map -> list and in the device details available in the AiMesh screen.
The full information about the device connections is available in the System Log - Wireless Log, which IMO is easier and more practical to use than checking one...