Poor 2.4GHz signal coverage on Zyxel Armor Z2 (NBG6817)

Have installed OpenWrt on my new NBG6817 for a while. Everything on wired connection works as expected, but I found out the my 2.4GHz signal cannot get past walls very well. The signals are fine inside my room, but as soon as I leave my room I lost most of the signal strength and have to switch to cellular data.

Is there a guide on how to configure wireless properly for ipq806x (which the router uses) in OpenWrt to improve signal coverage? And by the way, does WPA3 have impact on signal coverage? The access points are configured to be WPA2/WPA3 mixed mode, since some of my Android 10 devices are capable of using it.

Actually, my previous WRT3200ACM had better 2.4GHz coverage, so I ended up bringing back that device to cover the rest of the home area (with the WAN port connected to the Zyxel router through a pair of power line adapters). The problem, however, is that WRT3200ACM cannot use WPA3 and some IoT devices have issues connecting to it.

Signal strength of my nbg6718 is (more than) fine, on both bands. While wpa3 doesn't have any impact on range, I would still recommend to disable mixed mode for testing (either wpa2-only or wpa-only), as quite some android 10 and iOS don't like mixed mode.

The signal is indeed okay, but only while I'm inside the room. The 2.4GHz signal outside my room is very poor (usually only 2-3 of 5 bars) and devices would drop connection rather frequently.

As for 5GHz, such signals can never go past walls at all, and I know that well. It's been the case of all dual-band routers I've used.

As for WPA3, I know that some devices don't like mixed mode (namely 32-bit iOS devices, as tested), but most work without issues under such mode.

I tried adjusting the antennae's position but not quite effective, the router's placed at a high place, but WRT3200ACM had better 2.4GHz coverage even when placed at the same location.

On the other hand, not sure if having multiple APs may also impact the signal coverage as I had 3 APs assigned to 2.4GHz for multiple purposes, including a guest AP.