Zyxel NBG6817 Dual Boot Hardware switch

Not sure if you are specifically looking for a way to use a hardware switch or just for a way to switch to the alternative firmware. The latter can be done via a Luci package: https://openwrt.org/packages/pkgdata/luci-app-advanced-reboot and see the support thread here: https://forum.openwrt.org/t/web-ui-to-reboot-to-another-partition-for-linksys-zyxel-dual-partition-routers-and-to-power-off-power-down