Interesting problem - this feels similar to an issue I’ve been chasing down, in which devices on the same network (but different interfaces, e.g. WiFi vs. Ethernet) lose the ability to talk to one another.
In my case the setup is quite different (I’m using a Balance One, the devices are on VLAN) but the “no route to host” error, and the sporadic nature of the issue (it comes, and goes) is quite similar to what you are seeing.
Example thread: Outbound Policy + VLAN bug in 8.5 and 8.4.1
FYI, Peplink said they may have identified at least one issue, and it may be fixed in the latest beta.
Have you tried 8.5.3 beta firmware?