Yes, you can deactivate the Wi-Fi on the supplied router (Connect Box 3). You can find the exact instructions in the router's operating instructions or on the manufacturer's support page.
If you want to use your own router, you can switch the Connect Box 3 to bridge mode, provided this is supported by the network operator. Details can also be found in the instructions or on the support page.
The official Galaxus resources do not provide explicit instructions for deactivating Wi-Fi, but the Connect Box 3 offers this function as standard.