hey 
I'm having the following problems for a while now but I can't find a solution.
I have a proxmox host with pihole running in a LXC container. My router is a fritz.box, it's DNS is set to the container IP (see old request if it's unclear). Pihole is also set for some local DNS to my several services in proxmox and devices. I have a Synology DS215j in my network. The NAS has currently a static IP set within DSM, before it was DHCP with fixed IP by my fritzbox.
My problem is: The Synology becomes unreachable after some (random) amount of time. When I (re)start the NAS, I can mount it (NFS and AFP), I can access the web interface via IP and local DNS domain as well as SSH. But after a while, with no repeating pattern, it becomes unavailable for all those methods. This can be some minutes or several hours. The NAS however is running, the LAN led is on/blinking. However it is not shown in the fritz' overview of network devices. Pull and replug the Ethernet cable makes the NAS available again.
I'm asking the pihole forum, as I'm quite sure it's a DNS problem. The recent days, I disabled pihole and it the NAS was reachable for over 24 h.
I had unbound running, but I've disabled that again for test purposes (did not help)
Do you have any idea? I tried to find out something by the systemd/journalctl logs of my synology. However, the logs are not persistent, even though I modified the /etc/systemd/journald.conf config file.