I've just recently setup a Xiaomi Router 3G v2 (mt76) and a Belkin AC1750DB (ath10k-ct) both on recent OpenWRT builds in a 5GHz WPA2 PSK (CCMP) WDS AP/Client configuration.
They are only about 6m apart through a thin indoors brick wall but I can't run a cable unfortunately.
I'm using the Xiaomi to get internet on a desktop computer.
The setup was straight forward but the problem I'm having is that the TX rate is capped at a fixed 6 Mbit/s for some reason and literally kills upload speed, RX rate is fine.
I've tried swapping them and their roles (WDS Client/WDS AP), changing channels, band width, tx power decrease, more recent snapshot builds, even changed the Belkin's wireless driver to the ath10k flavour just in case but the only working solution I've found is to link them on the 2.4GHz band.
Could this be a wireless driver problem/bug or maybe just a compatibility thing?
Did you actually test the throughput (e.g. iperf3) or do you just believe the reading?
ath10k is almost a fullmac driver, meaning the firmware is solely in charge of most of the connection handling and doesn't always report back to the kernel (and 6 MBit/s is a base rate it reverts back to, in the absence of a real reading from the firmware).
I didn't do a specific iperf3 test but found that when trying to upload something to my local NAS or to Gdrive from the desktop, the transfer rate sits at 0MBit/s and then just fails with a network error (Windows). I couldn't even upload an Openwrt snapshot update to the Belkin over the link.
Any uploads crawl and finally fail, this only happens when it's a 5GHz WDS link, 2,4GHz is totally fine.
The Xiaomi in WDS AP mode and a client connected to the 5GHz radio (laptop) uploads are fine also.
After updating the Xiaomi to the latest snapshot (OpenWrt SNAPSHOT r11675-16b01fb1b9) and testing this again it seems to be more or less fixed, I spent hours trying to find the cause without luck the last few days .
Did some iperf rx/tx from the wds client cabled attached desktop computer to/from the Xiaomi wds ap and the results seem alright, could be better but I'm happy enough.