Hello,
You can achieve that by using Profiles scheduler functionality in System > Profiles > Scheduler. You will need to configure active WAN-LAN profile first, then create another profile that will have disabled WAN-LAN, but before disabling it make sure you configure the scheduler correctly and then disable everything. It may not work because the router will lose all connection to the internet and will not be able to check the time when it will need to switch profiles. It is recommended to keep at least mobile connection active even if it adds more power consumption.
Best regards.