Occasional problem with WiFi connection with Xiaomi Nano

So I installed this on Nano, https://downloads.openwrt.org/snapshots/trunk/ramips/mt7628/openwrt-ramips-mt7628-miwifi-nano-squashfs-sysupgrade.bin

The problem that I encountered is intermittent failures to access the Internet. Say I use my Android 4.4.2 phone and connect (no security), then I open Chrome and surf any websites - no problem. But when I switch my phone back on after idle for say 30 mins and access a website, all I got was loading and loading...until I got "You're offline" message from Chrome. Sometimes I do get to load the site after 2 mins of waiting, sometimes I don't.

This happens occasionally on these phones.
Samsung Galaxy S5 - Android 4.4.2 (most frequent)
LG-G3 - Android 4.4.2 (most frequent)
iPhone 6s - iOS 11.4 (less frequent)

It never seems to happen on these.
Sony Experia - Android 4.4.3
iPad 2 - iOS 9.3.5
Asus Zenbook - Windows 10

WiFi settings.
Channel: 2 (not used by other WiFis)
Mode: N
Width: 20MHz
Transit Power: Auto

Anyone has any ideas what's causing this issue?

Anything useful in the logfiles?

Oh course. Forgot that.. willcheck

To log, are you referring to using logd, https://wiki.openwrt.org/doc/howto/log.essentials

root@OpenWrt:~# ps | grep log
  418 root       892 S    /sbin/askfirst /usr/libexec/login.sh
 4570 root      1168 S    /sbin/logd -S 16
 4571 root      1336 S    /sbin/logread -f -F /tmp/system.log -p /var/run/logread.1.pid -S 16
 5745 root      1176 S    grep log

If so, my log config (etc/config/system) is as below. But I can't seem to get much info when my devices are not getting any webpages loaded.

config system
	option hostname 'OpenWrt'
	option ttylogin '0'
	option zonename 'UTC'
	option timezone 'GMT0'
	option log_size '16'
	option log_proto 'udp'
	option log_file '/tmp/system.log'
 	option log_type 'file'
	option conloglevel '8'
	option cronloglevel '5'

Log file:

Sat Jul  7 02:01:23 2018 daemon.info hostapd: wlan0: STA 84:38:38:03:60:7f IEEE 802.11: disassociated
Sat Jul  7 02:01:46 2018 daemon.notice hostapd: wlan0: STA 84:38:38:03:60:7f IEEE 802.11: did not acknowledge authentication response
Sat Jul  7 02:01:46 2018 daemon.notice hostapd: wlan0: STA 84:38:38:03:60:7f IEEE 802.11: did not acknowledge authentication response
Sat Jul  7 02:01:46 2018 daemon.info hostapd: wlan0: STA 84:38:38:03:60:7f IEEE 802.11: associated (aid 2)
Sat Jul  7 02:01:47 2018 daemon.info dnsmasq-dhcp[1064]: DHCPREQUEST(br-lan) 192.168.1.110 84:38:38:03:60:7f 
Sat Jul  7 02:01:47 2018 daemon.info dnsmasq-dhcp[1064]: DHCPACK(br-lan) 192.168.1.110 84:38:38:03:60:7f android-dafd2a5463007a92
Sat Jul  7 02:02:59 2018 daemon.info hostapd: wlan0: STA 84:38:38:03:60:7f IEEE 802.11: disassociated
Sat Jul  7 02:03:11 2018 daemon.notice hostapd: wlan0: STA 84:38:38:03:60:7f IEEE 802.11: did not acknowledge authentication response
Sat Jul  7 02:03:11 2018 daemon.info hostapd: wlan0: STA 84:38:38:03:60:7f IEEE 802.11: associated (aid 2)
Sat Jul  7 02:03:12 2018 daemon.info dnsmasq-dhcp[1064]: DHCPREQUEST(br-lan) 192.168.1.110 84:38:38:03:60:7f 
Sat Jul  7 02:03:12 2018 daemon.info dnsmasq-dhcp[1064]: DHCPACK(br-lan) 192.168.1.110 84:38:38:03:60:7f android-dafd2a5463007a92
Sat Jul  7 02:19:48 2018 daemon.info dnsmasq-dhcp[1064]: DHCPREQUEST(br-lan) 192.168.1.192 8c:3a:e3:18:b0:c9 
Sat Jul  7 02:19:48 2018 daemon.info dnsmasq-dhcp[1064]: DHCPACK(br-lan) 192.168.1.192 8c:3a:e3:18:b0:c9 android-82323d96558eea33

Here's an update. Apparently, this problem also occurs on more devices that I first thought. Maybe because now I've used more of these devices now. Here's the update.

This happens occasionally on these phones.
Samsung Galaxy S5 - Android 4.4.2 (most frequent)
LG-G3 - Android 4.4.2 (most frequent)
iPhone 6s - iOS 11.4 (less frequent)
iPad 2 - iOS 9.3.5 (quite frequent)
Asus Zenbook - Windows 10 (quite frequent)

It never seems to happen on these.
Sony Experia - Android 4.4.3 (maybe cos I hardly use this device)