@RecapsGallion Updated to firmware 2.1.4 Build 20230720 Rel.50798 and reset WAN and went from this - 

to this - 

Latency vastly improved but speed dropped after initial improvement.

Still experiencing disconnections from some active clients.

  @RecapsGallion 

What happens if you disconnect everything on the router except wan, connect a PC directly to the router and run a speed test.

  @MR.S works as it should - 

We have also run WinMTR with direct connection to ISP via PPPoE with good results, but when connected via LAN and ER605 we had 7% packet loss between PC and ER605 and 23% loss at fixed wireless dish.

  @RecapsGallion 

I think you probably need to look at the switcher/cabling, the router is probably not faulty :-) connect one switch at a time and see when the problem starts, then remove one cable at a time from the problem switch. it can be difficult to determine where the error is.

@MR.S when all 3 switches are connected to separate LAN ports on the ER605, and it is still disconnecting all devices across all three switches simultaneously, I don't see how it could be a single switch or cable that is faulty.  Also, most of the cables are new - replacing cables was the first step that I took to try to diagnose the fault.

  @RecapsGallion 

it is hard to say. but if the speed is normal when you disconnect all the switches, then I would start by looking elsewhere than on the router. I have 3 such routers myself and I have never experienced a similar problem.

  @MR.S it appears that the graph that I pasted above from the OC200 dashboard is not showing available speeds, but rather might be showing Mbps averaged over the 5 minute  window, which with lower traffic on a Sunday afternoon is a lower average.  With the switches back in the network, I've now run several speed tests showing the same speed as when the computer was connected directly to the ER605 with the switches unplugged (computer now connected through the TP-SG1016).

  @Clive_A @MR.S Thank you both for your comments.  

Since the firmware update to the latest Beta and resetting the WAN, I have experienced stable latency around 32msec, stable speeds of around 110+Mbps (on a 120Mbps connection) and no Ethernet disconnection events.

I will continue to monitor until next weekend and try the further diagnostics that you have suggested then, when other users will not be affected.

But so far, hopefully, it looks like the issue may be fixed.

@Clive_A @MR.S I spoke too soon about a solution.  

What do you make of the following graph - the ER605 dropping all client connections (including the OC200) this afternoon.  This was not an ISP failure; no failover to WAN2.  All 3 LAN ports dropped at the same times.