Limit client to individual gateway within gateway group

[spacecase-25]

Hello, I have a set up involving a remote power switch that is set up to automatically restart my cable modem if the connection goes down.  However, I also have a backup 5G modem that is included in my grouped gateway.  What I would like to do is limit the IP address of the remote power switch to only using the cable modem as an internet gateway, that way it will still restart the cable modem if the connection goes down, but the backup 5G connection is still working.  I've tried various combinations of firewall rules, but the remote power switch keeps having access to the internet when the cable modem is unplugged. 

How can I do this?  I do not have the option to put the remote power switch on a vlan, as I do not have a managed switch.

Thanks.

[bartjsmit]

Set a default gateway on the (device that controls the) power switch to use the 5G modem.