How long does it take? Your issue could be this, Xiaomi Wifi R3P pro - 19.07.x / Permanent Wifi clients unreachable after 1 day - Installing and Using OpenWrt / Network and Wireless Configuration - OpenWrt Forum
Same with vanilla openwrt and @db260179 builds. I can't reach my local devices after sometime, contantly pinging the ip's brings them back up for some reason. I don't have this issue with official firmware.