@Auren 

Hello, sorry to hear that you have the trouble with the product.

The issue is very strange since the extender connects your main host router first, then extends the Wi-Fi. Once your host router works fine first, then the extender will work fine.

But you said the extender's Wi-Fi is working well, your host main router's Wi-Fi has something issue.

I have checked the firmware version of your extender and it is a little old.

It is suggested that you can follow the instruction to upgrade the firmware version to the latest one first. https://www.tp-link.com/support/faq/1416/

Then reconfigure the extender again to check if the same issue happens again.

Note: We suggest you can change the different Wi-Fi name for the extender from the host router.

If the same issue happens again, please check the lights' status on the extender.

The model number of main host router? If the main router and extenders have the same Wi-Fi name or not.

Then please connect the router's Wi-Fi and follow the instruction to check the IP address, default gateway on your PC. 

https://www.tp-link.com/support/faq/838/

Also, please connect the extender's Wi-Fi and follow the instruction to check the IP address, default gateway on your PC.

BTW, may I know when and where you bought the extender? When did the issue begin?

Look forward to your reply.

Good day!