OK, so you have a few variables introduced at the same time. The best thing you can do is test one part at a time.
You say that your PC fails to access the internet when this problem is present, so let's use that as the test. I'm going to assume your PC is running Windows, so tap the start button in the bottom left of your screen and start typing on your keyboard:
command prompt
You'll see a black rectangle app icon, and an app called "Command Prompt" - select it. In the black box that appears type the following command and hit enter:
ipconfig /renew
You'll see a bunch of text appear - you can ignore it for now.
After each test, run the above command and then try to access the internet from your browser on that PC as normal.
My recommended order of events would be:
1. Plug everything in - i.e. the situation that normally breaks it. Wait for the TP-Link to connect, then run the above command and see if the internet is broken. If it is, then continue with step 2 - otherwise, my idea is blown, and we need a new plan.
2. Unplug everything to put it back to working state again, and run the above commands to confirm it's working.
3. Plug in just the TP-Link master device near your hub, wait a minute for it to boot, then test.
4. Plug in the extender you're using for CCTV, but don't connect the CCTV to it. Wait a minute for it to boot and connect, then test.
5. Plug the CCTV into the extender, and test - confirm that both the PC and CCTV are working.
6. Plug in the cabin extender. Wait a minute for it to boot and connect, then test.
If it still hasn't failed at this point and there are any other devices that you're also connecting, add them one at a time before testing.
