@Kevin_Z We've managed to solve this issue. I've used Google's DNS addresses on both of IPv4 and IPv6. The IPv4 is working fine but the IPv6 is not. So we changed the following setting in the routers...

@Kevin_Z I've got the email and replyed, also sent a screenshot and the system log. Hope it helps. Another thing I found out that with the latest firmware (with the "Poor network." message) TP-Link...

@Kevin_Z That's Google's free DNS addresses, I'm using those from day 1. Also tried Cloudflare DNS addresses but the issue still exists.

@ichthyosaurus No, it was the right firmware, you router will find it eventually. Mine did too, but I won't upgrade again. Maybe if another firmware pops up I will give it a try.

@ichthyosaurus No, I'm using the previous firmware.

Anyone?