Check whether the data traffic is too large, check the CPU Usage and the Logs when you still can access the web-GUI, whether has the error information.

Please refer to the following configuration. If it can solve your problem, please choose this reply as the best solution. VLAN1: Port1: tagged Port2-5,7-8: untagged VLAN100: Port1: tagged Port6:...

Check the negotiation speed between the switch and the router, Check the negotiation speed between the switch and the end devices.

Connect the PC to the switch directly and use the latest version of Unmanaged Pro Utility.

Maybe you can try to upgrade the frimware and you can refer to the FAQ. https://www.tp-link.com/us/faq-1057.html

You don't need to care about the multi-nat of the router, I think you are use the TL-R480T+ V9, just choose the WAN port which used to access the internet. You can connect a PC to the guest network...