Sort of the same problem…swapping out two PCIe network adapter cards wouldn’t give me above ca. 500mbps down on my Win10 Pro desktop, even though my laptop managed ca. 925 on a 800 plan. Finally bought and installed a TP-Link 10/100/1000 PCIe card, and - presto!, got my speed. Can’t explain why, but couldn’t argue with the result.
Fully second that…had an older tplink A7 that was fine for previous 200 mbps down speed tier, but simply flopped when upgraded to 800 mbps speed…installed the AX55 (AX3000) as primary router, and every device w/ gigabit NIC now records >900 mbps down. Couldn’t be more pleased for a $109 investment.