Home Network Setup: Cable Modem connects to TP-Link Archer A9. Archer A9 broadcasts wifi and has devices connected via Ethernet. Wired devices includes three switches which then have various devices connected to them: computers, TVs, gaming consoles, etc.
Problem: All was working fine until yesterday when the Internet throughout entire house went out. Did all of the normal devices reboots; nothing worked. Tested modem by connecting directly to it via Ethernet; Internet worked fine. Connected Archer A9 to modem and verified Wifi devices Internet worked. Plugged one switch in at a time. Switches #1 and #2 Internet worked fine. As soon as Switch #3 was connected, the problem happened again where the entire house (both wifi and wired) all lost Internet. Assuming it was a faulty switch, I swapped it out for an old router that has been configured to work like a switch (DHCP disabled) and it happened again, all lost Internet. Assumption is that the switch #3 was not faulty and it might be one of the devices connected to it.
Question: What is going on? Could a device connected to switch #3 cause this type of disruption that causes all devices to lose Internet connectivity? Any way to resolve this problem?