Indicate an external state to a router

Is there some "light switch" or anything else what can indicate its state to a router.

In more detail:
I am searching for a simple Wi-Fi or Ethernet device, presence of which in the network can be quickly turned on and off in simple way – physically – to make a router recognize some external condition.

For example: When the device is switched on, it can reply to ping at static IP address 192.168.31.31 and the MikroTik router (using its scripting language) knows that in turn it should enable for example guest Wi-Fi. Once the device is switched off, ping of 192.168.31.31 no longer returns a reply so the router will disable the feature.

Ideally it should be a passive device or one with very low power consumption, low maintenance (i.e. not an older mobile phone which needs to be maintained charged and has a lengthy bootup/shutdown). The device does not need to be portable. If plugging/unplugging an Ethernet cable somewhere solves the problem, it would be also acceptable.