Problem with Google Home/Assistant accessing Home Assistant on VPN

I recently made the switch to put my entire network/IT infrastructure behind a VPN, including my Home Assistant.
The VPN is activated on my router, a Cisco RV340.

Everything kept working (except for some internal addresses that now need to directly use IP addresses), except for the link between Google Home/Assistant and Home Assistant.
Google Home/Assistant can still, for instance, control my Sonos Devices, but lights accessed through Home Assistant don’t work where they did before.
Google Assistant gives the error message:
“I’m afraid I couldn’t reach my test app”

Any clue on how to solve this?