@radishman, I’m back with the results.
- Switching to the latest snapshot version of openwrt based on ath9 fixed the issue with Wi-Fi. Wi-Fi is fast and stable now! This could be the end of story however WAN link turned out to be unstable this time. ETH1 interface keeps up and down frequently.
- I dug a bit into the issue with WAN link and found that reverting a couple of commits from the master branch and rebuilding the image solves the issue with WAN too.
Result: Now my GL-AR300M works without any issues.