Off-Site devices connected via Wireguard tunnel can be configured but then are "unavailable"

I am new to HA but already amazed by it! I have installed HA on a Synology NAS and started to write some first automations.
The issue I encountered: I would like to also monitor Shelly devices in my holiday house. The two locations are permanently linked through a Wireguard VPN tunnel established by two Fritz!Box routers, thus all devices appear on two separate subnets (eg., 192.168.1.xxx and 192.168.2.xxx). In HA, I could configure the Shelly devices at my holiday house by providing their (fix) IP-addresses on the second subnet. They appeared on the dashboard, but only one datapoint (temperature) was read. Subsequently, they appear as “not available”.
Any clue what is missing? I have tried to open port 80 (used by Shelly devices) in the NAS firewall, but this didn’t help. I have searched this forum and found this similar topic, but the solution doesn’t seem to apply to my case.
Any help is much appreciated - thanks in advance!