WLAN errors

> [...] believe its due to problems with modem cm1200 configuration with
> COX Communication switch. [...]

   Why?  What is "COX Communication switch" when it's at home?

> [WLAN access rejected: incorrect security] from MAC address ec:f0:0e:7d:4c:57, [...]

   I'd expect that message to be caused by some wireless gizmo which is
trying to connect to your router using a good SSID and bad passphrase.

   A CM1200 is a modem, and has no wireless capability.  It would not be
my first suspect in a complaint about a failing wireless connection.

> [...] What can I do about it?

   Program the mystery gizmo with valid wireless-network credentials?

Do you have other devices than the XR500 plugged into the CM1200? 

It has 4x ports but cox only assigns 1 public IP address. 

So other devices connected directly to the CM1200 won't work. (and if they do, the xr500 wouldn't work)

An OUI loopup comes up with the vendor AboCom. Do you have an old or off-brand smart home products such as smart outlets, leak detectors, wireless IP cameras, or any device likely to be sold on sites like aliexpress?

If not, check to ensure they you do not have the same SSID as someone else in range where their smart home products may try to connect to your network.