@CSH if you connect a switch to the modem with the modem running in its default Gateway mode and run into problems at some point, simply disconnect the switch and see if the problem still exists without the switch. If so, "Hello Rogers, over to you .....". If not, then the issue is caused by something within your home network.
A switch is just a dumb device more or less, as long as it is an unmanaged switch. The only way it would cause you problems if it stopped work alltogether. And that is highly unlikely.
Just get a switch, multiple lan rj45 on a modem/router is just a switch built in.