I'm facing the very same issue.... running version is 7.7
I have tested the setup by drop in replacement of the existing cheap AP - place, obstacles, distance and devices operating are exactly the same. Environment is the same.
I'm so much disappointed with AX3 performance... jaw dropped... testing stations never reaches more than 300 Mbit/s real speed to NAS or Internet (AP-> 1G LAN ->virtual pfSense router on i5-12600 server, so fast as it can be-> 1G internet) and it never ever negotiates more link speed than 400 Mbit.... it is also very unstable, drops to very low MCS and low speeds (80-108 Mbps)
Interesting thing that this 300 Mbit seem to be some cap as it mostly equal 300 sharp.
My testing laptop and phone (MacBook Pro 2019, 802.11ac 3/MIMO, iPhone 11 which is 802.11ax 2/MIMO) are about 2.5 meters away from the AP in a straight line. Last machine tested was win11 PC with Intel ax 210 nic. This one stands closest to the AP - 50 cm, antena to antena.
Usually, when stations moved away even slightly from straight line, the connection drops parameters even harder, then stations decide to move to ... further AP which is 6-7 meters away behind brick wall and wooden door, it is Asus AX55 (same radio configuration as AX3)
All of this happens in 5Ghz band. When it comes to 2Ghz band, performance is even worse.... devices close to the AX3 like old printer, old AV receiver, some ESP controllers from IoT can't catch the radio and moving to Asus. Unusable even in the same room!
Config - rather simple, but adding channels, channel width etc. gave no gain, so I decided to left it as default as possible but still analogue to other devices I use:
set [ find default-name=wifi2 ] channel.band=2ghz-ax .skip-dfs-channels=\
disabled .width=20/40mhz configuration.country=Poland .mode=ap .ssid=\
hardc0re_2G disabled=no name=wifi-2G security.authentication-types=\
set [ find default-name=wifi1 ] channel.band=5ghz-ax configuration.country=\
Poland .mode=ap .ssid=hardc0re_5G disabled=no name=wifi-5g \
ccmp,gcmp,ccmp-256,gcmp-256 .group-encryption=ccmp .management-encryption=\
Channels are different on both APs, for 2 and 5 Ghz bands. There are no other interferences I can notice.
Funny thing that I have replaced .... cheap-of-cheapest Xiaomi AX3000 Mesh system box, which barely have any configuration. Very basic thing. On this one, I got full possible negotiated 802.11ac speeds (866 or 10xx something, but rarely) on MBP, and 1200 on iPhone 11. Xiaomi AX3000 has no external antenas. 2Ghz band was giving full range for older devices too...
It looks like some flaw somewhere - usually, from my experience, the problem might be with hardware radio path eg. some shorting to ground, internal interference, problems with antenas and connectors... just guessing. But this behavior is very strange.
What I have checked seems ok. Received power and noise floor for 5Ghz on the stations seem quite good but 2Ghz are terribly bad and very unstable (jumps from very low to good values constantly)
Any ideas what can be wrong with this device? I'm very very surprised by its poor wifi performance, would never expect that from Mikrotik box.....