@Korgua 

I don't think wireless settings will affect ping test from LAN side.

However you may try replacing the Ethernet cables between your LAN network and the PoE injector(of the CPE). Under my test the LAN port of the PoE injector is not good, can only work with short Ethernet cables

  @Somnus 

Having left the system for a few hours all the LAN side pings dropped to 1-10ms - still not perfect but not in the 500ms+

This lasted for 48 hours and then one of the CPE510 pings increased to 500ms+.  All other LAN pings were 1-10ms at the time.  This issue last 15 minutes or so and remedied itself.  A few minutes later another of the CPE510 pings increased to 500ms+

If you attempt to web manage the CPE at the time the pings are slow then you often get no web page or "failed to get data" or very slow web pages in the CPE web GUI - suggesting to me the CPU is the bottleneck.

Cables have been swapped out and the LAN side are only 2m patch leads.

  @Korgua 

Could there be some influence between CPE510s?(like high traffic etc.)

Are you able to power off two CPE510, only leave one CPE510 working and monitor if it can work stable?

Or maybe take down a pair of CPE510-CPE710 and run the test in-door? (don't connect this test pair of CPE to the same network as the others)

  @Somnus 

How would the traffic to each CPE510s influence each other?

The remote CPE710 only have cameras connected so the traffic is a continual stream of 8-15Mbits.  This seems well within the capabilities of the WLANs and LANs.

I have no access to site at this time.  The only test I could perform would be to use the webgui on a CPE510 and disable the AP, thereby dropping all traffic to the remote CPE710.

As for bench testing - this is not possible, for the same reason above.  I have a call logged with TP-Link support.