This router is being affected by the same issue described here which hinders 802.11ax performance with obstacles (such as brick walls) in the WiFi path:
(tested with an iPhone 13Pro and a snapshot build OpenWrt SNAPSHOT r24769-96580500d2 I did from 2024-01-06 master)
I will try to manually apply the solution of disabling 160Mhz mode (that solved the issue in other devices) to see if it makes any difference.
@dimfish if I confirm disabling 160Mhz mode fixes the issue, it would be good to include a patch to disable 160MHz in the AX3000T build similar to this one for mt7915.