5-port switch causing internet disconnection

Good afternoon everyone,

I recently bought a tp link tl-sg1005d switch as our current modem/routers Ethernet ports are full.

The modem/router combo is utilized for our credit card machine, laptop computer, and a few other misc items. We purchased 3 new cash registers that need to be hard wired, so I bought the switch. The only items plugged into the switch are the 3 cables to the registers.

When I plug the switch into the modem/router combo, the internet immediately goes down for all devices and the light on the router goes out until the switch is unplugged.

For reference, the instructions called for the switch to be plugged into the Lan/Wan port on the router, so i tried that, as well as just plugged it into a normal port, both of which take the internet down.

Any advice is helpful! Thank you.