HomeKit bridge not accessible on secondary router

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HomeKit Bridge not accessible on secondary router

So, here’s my setup:

  • Primary router (DIR-610) (connected to the Internet)
    • Secondary router (DIR-815) (works as an access point, same network, LAN to LAN connection)
      • Raspberry Pi running Home Assistant with the HomeKit integration enabled (connected through Ethernet)

When I connect things like this, the bridge is accessible on both routers (using an iPhone or a MacBook over Wi-Fi). This image shows what the Bonjour Discovery list looks like: Imgur

However, when I connect the Raspberry Pi to the primary router instead, the bridge is accessible only when connected to the primary Wi-Fi. On the secondary router, only the Home Assistant dashboard is accessible, and Home and Home Assistant Bridge disappear completely from Bonjour Discovery: Imgur

I’ve already tried changing some settings (like turning multicasts on/off, opening ports, etc.) on the secondary router (The primary router is blocked by my ISP. Contacting them is my last resort).

Has anyone ever faced something liked this? Suggestions? Is it possible that my router is somehow blocking port 51827?