LAN stops working every now and then

My SmokePing is running now for 4 days without a single lost packet on LAN. I can also say for sure that I never had any outages that lasted for more than 10 to 15 minutes because one of my monitoring servers is behind the router. So, I am afraid, I cannot help debug this problem. Seems my router is not affected.

Update 2017-05-24 12:28: Added specs:

I am running LEDE Reboot 17.01.1 r3316-7eb58cf109, essentially default settings apart from: LAN is, router has static IPs, NAT leakage workaround, WLAN, odhcpd, and some IPv6 features are disabled.