@RobWilcox Is there any warning or alert can be found on the Controller Log?

The "Online" status only shows if the controller can communicate with the Cloud server, so it may not be related to the local connection, but actually Internet access of the controller. If possible, try link up this OC200 to another Internet source to check if it can be online stable.

  @Somnus 

I've just looked at the logs for the last 24 hours (where I've received about 15-20 'offline' notifications).

There are no warnings. 

I read through most if not all of the entries (as the network is not busy yet). None of them relate to not being able to connect to the cloud.

  @RobWilcox If your router also tplink? Check is there any other devices use the same IP as the router, specially the switch. Some switches may also use 192.168.0.1 as default IP.

  @Somnus 

The router is a TP-Link Archer MR600 v2 (4G)

It has the IP address of 192.168.1.1.

The OC200 has the IP address of 192.168.1.4

The DHCP range is 192.168.1.25-onwards.

There isn't anything using the same IP address, unfortunately.

  @RobWilcox Could that be the 4G network issue? I know some 4G network will need to change IP frequently and that may cause a disconnection to the cloud server.

On your router you should be able to see the WAN/Internet IP address, is there any change on that IP?

  @Somnus 

The IP address is exactly the same as 1 week ago.

I'm at the location right now, and the cloud controller says 'offline' for 2-3 minutes, yet the internet connection shows no drop in packets or connectivity.

  @RobWilcox Then I have no idea now. You may need to capture package from router WAN, to see if there is any problem on communication to the cloud server.

You can access the controller in local LAN so I don't think it is an OC200 issue, but anyway you may try contact tplink and ask for support.