Can no longer ping IP

I have an RB800 router with two WAN connections shared with 4 WLAN connections.
The two connections are balanced based on LAN IP.
At random, one of the WAN IPs will no longer respond to Ping and therefore DUDE shows the router is down.

What would cause the WAN IP to stop responding to pings?
I checked with the ISP and they found no problems with the gateway. It also worked just fine for more than 6 months and now just stopped.
:unamused: