After an hour of investigation I have found that this issue is related to
- [Omnia] kernel's bridge FDB not reconciling switch's ATU (#165) · Issues · Turris / Turris OS / Turris Build · GitLab
- TO LAN switch or bridge blocks DHCP replies intermittently
- Lan ports aren't accessible for WiFi client for the first 250-330 seconds after connection
- OMNIA: Vlan on DSA port breaks arp responses (TOS 4.0.5)
- Access point configuration problem - Network and Wireless Configuration - OpenWrt Forum