I have a client with an RV340 router that works well most of the time. However, every so often the WAN disconnects for about 25-40 seconds, and then it's back up. The ISP Swears it is not them. For the most part, this goes unnoticed unless someone is connected to the VPN when it happens and they get disconnected. It is also random in how often it occurs. Sometime once a day or sometimes 10 times in 10 minutes.
Sample log messages. IP changed to protect the innocent.
2019-10-15T19:34:09-04:00 <info>network: wan1: interface is now down
2019-10-15T19:34:34-04:00 <info>network: wan1: interface is now up with 192.168.16.254
2019-10-15T19:35:06-04:00 <error>smart_agent: SA-ERROR: Recover OOC entry time and grace expiration time in TS failed.
Router is on the most recent firmware. I just updated that a couple of weeks ago to make sure that wasn't the problem.