OpenWRT WIFI AP mode, too long to be found on WIFI scan

Hi everybody and happy new year,

I have a small issue with OpenWRT device which is being used mainly as VPN gateway and wifi access point. When I do a wifi scan from any device to list my OpenWRT networks (no matter if the client is a UNIX/Windows computer, smartphone...) I see every other wifi network, but my 2.4 Ghz and 5Ghz Openwrt networks always take a very long time to appear in the wifi found list. No matter if I'm one meter away or in a remote room with weaker signal, it just takes an abnormal long time to be found (not timed, but something about 30 seconds where any other neighbour wifi is listed in few seconds)

My atheros based WDR3600 is currently at 19.07.7 and I'm trying to avoid updating to 21 if not absolutely necessary, because USB extroot plus many services and configs were hard to success.

The issue I'm describing was not present in the same device when I had the device working back in 15.x Chaos Calmer, with same wifi configs. After giving up on tweaking wifi settings and ruling out clients scanning problem, I concluded the issue is in the OpenWRT serving as access point.

I would appreciate if someone can point me to any parameter or setting that could be producing this issue. Appart from this, once logged in to the wifi, everything works fine.

Thank you very much in advance.